pcre2api man page

This page is part of the PCRE2 HTML documentation. It was generated
automatically from the original man page. If there is any nonsense in it,
please consult the man page, in case the conversion went wrong.

These functions became obsolete at release 10.30 and are retained only for
backward compatibility. They should not be used in new code. The first is
replaced by pcre2_set_depth_limit(); the second is no longer needed and
has no effect (it always returns zero).

These functions provide a way of converting non-PCRE2 patterns into
patterns that can be processed by pcre2_compile(). This facility is
experimental and may be changed in future releases. At present, "globs" and
POSIX basic and extended patterns can be converted. Details are given in the
pcre2convert
documentation.

There are three PCRE2 libraries, supporting 8-bit, 16-bit, and 32-bit code
units, respectively. However, there is just one header file, pcre2.h.
This contains the function prototypes and other definitions for all three
libraries. One, two, or all three can be installed simultaneously. On Unix-like
systems the libraries are called libpcre2-8, libpcre2-16, and
libpcre2-32, and they can also co-exist with the original PCRE libraries.

Character strings are passed to and from a PCRE2 library as a sequence of
unsigned integers in code units of the appropriate width. Every PCRE2 function
comes in three different forms, one for each library, for example:

The UCHAR types define unsigned code units of the appropriate widths. For
example, PCRE2_UCHAR16 is usually defined as `uint16_t'. The SPTR types are
constant pointers to the equivalent UCHAR types, that is, they are pointers to
vectors of unsigned code units.

Many applications use only one code unit width. For their convenience, macros
are defined whose names are the generic forms such as pcre2_compile() and
PCRE2_SPTR. These macros use the value of the macro PCRE2_CODE_UNIT_WIDTH to
generate the appropriate width-specific function and macro names.
PCRE2_CODE_UNIT_WIDTH is not defined by default. An application must define it
to be 8, 16, or 32 before including pcre2.h in order to make use of the
generic names.

Applications that use more than one code unit width can be linked with more
than one PCRE2 library, but must define PCRE2_CODE_UNIT_WIDTH to be 0 before
including pcre2.h, and then use the real function names. Any code that is
to be included in an environment where the value of PCRE2_CODE_UNIT_WIDTH is
unknown should also use the real function names. (Unfortunately, it is not
possible in C code to save and restore the value of a macro.)

If PCRE2_CODE_UNIT_WIDTH is not defined before including pcre2.h, a
compiler error occurs.

When using multiple libraries in an application, you must take care when
processing any particular pattern to use only functions from a single library.
For example, if you want to run a match using a pattern that was compiled with
pcre2_compile_16(), you must do so with pcre2_match_16(), not
pcre2_match_8() or pcre2_match_32().

In the function summaries above, and in the rest of this document and other
PCRE2 documents, functions and data types are described using their generic
names, without the _8, _16, or _32 suffix.

PCRE2 has its own native API, which is described in this document. There are
also some wrapper functions for the 8-bit library that correspond to the
POSIX regular expression API, but they do not give access to all the
functionality of PCRE2. They are described in the
pcre2posix
documentation. Both these APIs define a set of C function calls.

The native API C data types, function prototypes, option values, and error
codes are defined in the header file pcre2.h, which also contains
definitions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release numbers
for the library. Applications can use these to include support for different
releases of PCRE2.

In a Windows environment, if you want to statically link an application program
against a non-dll PCRE2 library, you must define PCRE2_STATIC before including
pcre2.h.

The functions pcre2_compile() and pcre2_match() are used for
compiling and matching regular expressions in a Perl-compatible manner. A
sample program that demonstrates the simplest way of using them is provided in
the file called pcre2demo.c in the PCRE2 source distribution. A listing
of this program is given in the
pcre2demo
documentation, and the
pcre2sample
documentation describes how to compile and run it.

The compiling and matching functions recognize various options that are passed
as bits in an options argument. There are also some more complicated parameters
such as custom memory management functions and resource limits that are passed
in "contexts" (which are just memory blocks, described below). Simple
applications do not need to make use of contexts.

Just-in-time (JIT) compiler support is an optional feature of PCRE2 that can be
built in appropriate hardware environments. It greatly speeds up the matching
performance of many patterns. Programs can request that it be used if
available by calling pcre2_jit_compile() after a pattern has been
successfully compiled by pcre2_compile(). This does nothing if JIT
support is not available.

More complicated programs might need to make use of the specialist functions
pcre2_jit_stack_create(), pcre2_jit_stack_free(), and
pcre2_jit_stack_assign() in order to control the JIT code's memory usage.

JIT matching is automatically used by pcre2_match() if it is available,
unless the PCRE2_NO_JIT option is set. There is also a direct interface for JIT
matching, which gives improved performance at the expense of less sanity
checking. The JIT-specific functions are discussed in the
pcre2jit
documentation.

A second matching function, pcre2_dfa_match(), which is not
Perl-compatible, is also provided. This uses a different algorithm for the
matching. The alternative algorithm finds all possible matches (at a given
point in the subject), and scans the subject just once (unless there are
lookaround assertions). However, this algorithm does not return captured
substrings. A description of the two matching algorithms and their advantages
and disadvantages is given in the
pcre2matching
documentation. There is no JIT support for pcre2_dfa_match().

In addition to the main compiling and matching functions, there are convenience
functions for extracting captured substrings from a subject string that has
been matched by pcre2_match(). They are:

The PCRE2 API uses string lengths and offsets into strings of code units in
several places. These values are always of type PCRE2_SIZE, which is an
unsigned integer type, currently always defined as size_t. The largest
value that can be stored in such a type (that is ~(PCRE2_SIZE)0) is reserved
as a special indicator for zero-terminated strings and unset offsets.
Therefore, the longest string that can be handled is one less than this
maximum.

PCRE2 supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (linefeed)
character, the two-character sequence CRLF, any of the three preceding, or any
Unicode newline sequence. The Unicode newline sequences are the three just
mentioned, plus the single characters VT (vertical tab, U+000B), FF (form feed,
U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
(paragraph separator, U+2029).

Each of the first three conventions is used by at least one operating system as
its standard newline sequence. When PCRE2 is built, a default can be specified.
The default default is LF, which is the Unix standard. However, the newline
convention can be changed by an application when calling pcre2_compile(),
or it can be specified by special text at the start of the pattern itself; this
overrides any other settings. See the
pcre2pattern
page for details of the special character sequences.

In the PCRE2 documentation the word "newline" is used to mean "the character or
pair of characters that indicate a line break". The choice of newline
convention affects the handling of the dot, circumflex, and dollar
metacharacters, the handling of #-comments in /x mode, and, when CRLF is a
recognized line ending sequence, the match position advancement for a
non-anchored pattern. There is more detail about this in the
section on pcre2_match() options
below.

The choice of newline convention does not affect the interpretation of
the \n or \r escape sequences, nor does it affect what \R matches; this has
its own separate convention.

In a multithreaded application it is important to keep thread-specific data
separate from data that can be shared between threads. The PCRE2 library code
itself is thread-safe: it contains no static or global variables. The API is
designed to be fairly simple for non-threaded applications while at the same
time ensuring that multithreaded applications can use it.

There are several different blocks of data that are used to pass information
between the application and the PCRE2 libraries.

The compiled pattern

A pointer to the compiled form of a pattern is returned to the user when
pcre2_compile() is successful. The data in the compiled pattern is fixed,
and does not change when the pattern is matched. Therefore, it is thread-safe,
that is, the same compiled pattern can be used by more than one thread
simultaneously. For example, an application can compile all its patterns at the
start, before forking off multiple threads that use them. However, if the
just-in-time (JIT) optimization feature is being used, it needs separate memory
stack areas for each thread. See the
pcre2jit
documentation for more details.

In a more complicated situation, where patterns are compiled only when they are
first needed, but are still shared between threads, pointers to compiled
patterns must be protected from simultaneous writing by multiple threads, at
least until a pattern has been compiled. The logic can be something like this:

Get a read-only (shared) lock (mutex) for pointer
if (pointer == NULL)
{
Get a write (unique) lock for pointer
pointer = pcre2_compile(...
}
Release the lock
Use pointer in pcre2_match()

Of course, testing for compilation errors should also be included in the code.

If JIT is being used, but the JIT compilation is not being done immediately,
(perhaps waiting to see if the pattern is used often enough) similar logic is
required. JIT compilation updates a pointer within the compiled code block, so
a thread must gain unique write access to the pointer before calling
pcre2_jit_compile(). Alternatively, pcre2_code_copy() or
pcre2_code_copy_with_tables() can be used to obtain a private copy of the
compiled code before calling the JIT compiler.

Context blocks

The next main section below introduces the idea of "contexts" in which PCRE2
functions are called. A context is nothing more than a collection of parameters
that control the way PCRE2 operates. Grouping a number of parameters together
in a context is a convenient way of passing them to a PCRE2 function without
using lots of arguments. The parameters that are stored in contexts are in some
sense "advanced features" of the API. Many straightforward applications will
not need to use contexts.

In a multithreaded application, if the parameters in a context are values that
are never changed, the same context can be used by all the threads. However, if
any thread needs to change any value in a context, it must make its own
thread-specific copy.

Match blocks

The matching functions need a block of memory for storing the results of a
match. This includes details of what was matched, as well as additional
information such as the name of a (*MARK) setting. Each thread must provide its
own copy of this memory.

Some PCRE2 functions have a lot of parameters, many of which are used only by
specialist applications, for example, those that use custom memory management
or non-standard character tables. To keep function argument lists at a
reasonable size, and at the same time to keep the API extensible, "uncommon"
parameters are passed to certain functions in a context instead of
directly. A context is just a block of memory that holds the parameter values.
Applications that do not need to adjust any of the context parameters can pass
NULL when a context pointer is required.

There are three different types of context: a general context that is relevant
for several PCRE2 operations, a compile-time context, and a match-time context.

The general context

At present, this context just contains pointers to (and data for) external
memory management functions that are called from several places in the PCRE2
library. The context is named `general' rather than specifically `memory'
because in future other fields may be added. If you do not want to supply your
own custom memory management functions, you do not need to bother with a
general context. A general context is created by:

Whenever code in PCRE2 calls these functions, the final argument is the value
of memory_data. Either of the first two arguments of the creation
function may be NULL, in which case the system memory management functions
malloc() and free() are used. (This is not currently useful, as
there are no other fields in a general context, but in future there might be.)
The private_malloc() function is used (if supplied) to obtain memory for
storing the context, and all three values are saved as part of the context.

Whenever PCRE2 creates a data block of any kind, the block contains a pointer
to the free() function that matches the malloc() function that was
used. When the time comes to free the block, this function is called.

A compile context is required if you want to provide an external function for
stack checking during compilation or to change the default values of any of the
following compile-time parameters:

What \R matches (Unicode newlines or CR, LF, CRLF only)
PCRE2's character tables
The newline character sequence
The compile time nested parentheses limit
The maximum length of the pattern string
The extra options bits (none set by default)

A compile context is also required if you are using custom memory management.
If none of these apply, just pass NULL as the context argument of
pcre2_compile().

A compile context is created, copied, and freed by the following functions:

A compile context is created with default values for its parameters. These can
be changed by calling the following functions, which return 0 on success, or
PCRE2_ERROR_BADDATA if invalid data is detected.

int pcre2_set_bsr(pcre2_compile_context *ccontext, uint32_t value);

The value must be PCRE2_BSR_ANYCRLF, to specify that \R matches only CR, LF,
or CRLF, or PCRE2_BSR_UNICODE, to specify that \R matches any Unicode line
ending sequence. The value is used by the JIT compiler and by the two
interpreted matching functions, pcre2_match() and
pcre2_dfa_match().

As PCRE2 has developed, almost all the 32 option bits that are available in
the options argument of pcre2_compile() have been used up. To avoid
running out, the compile context contains a set of extra option bits which are
used for some newer, assumed rarer, options. This function sets those bits. It
always sets all the bits (either on or off). It does not modify any existing
setting. The available options are defined in the section entitled "Extra
compile options"
below.

This sets a maximum length, in code units, for any pattern string that is
compiled with this context. If the pattern is longer, an error is generated.
This facility is provided so that applications that accept patterns from
external sources can limit their size. The default is the largest number that a
PCRE2_SIZE variable can hold, which is effectively unlimited.

This specifies which characters or character sequences are to be recognized as
newlines. The value must be one of PCRE2_NEWLINE_CR (carriage return only),
PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the two-character
sequence CR followed by LF), PCRE2_NEWLINE_ANYCRLF (any of the above),
PCRE2_NEWLINE_ANY (any Unicode newline sequence), or PCRE2_NEWLINE_NUL (the
NUL character, that is a binary zero).

A pattern can override the value set in the compile context by starting with a
sequence such as (*CRLF). See the
pcre2pattern
page for details.

When a pattern is compiled with the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE
option, the newline convention affects the recognition of white space and the
end of internal comments starting with #. The value is saved with the compiled
pattern for subsequent use by the JIT compiler and by the two interpreted
matching functions, pcre2_match() and pcre2_dfa_match().

This parameter ajusts the limit, set when PCRE2 is built (default 250), on the
depth of parenthesis nesting in a pattern. This limit stops rogue patterns
using up too much system stack when being compiled. The limit applies to
parentheses of all kinds, not just capturing parentheses.

There is at least one application that runs PCRE2 in threads with very limited
system stack, where running out of stack is to be avoided at all costs. The
parenthesis limit above cannot take account of how much stack is actually
available during compilation. For a finer control, you can supply a function
that is called whenever pcre2_compile() starts to compile a parenthesized
part of a pattern. This function can check the actual stack size (or anything
else that it wants to, of course).

The first argument to the callout function gives the current depth of
nesting, and the second is user data that is set up by the last argument of
pcre2_set_compile_recursion_guard(). The callout function should return
zero if all is well, or non-zero to force an error.

The match context

A match context is required if you want to:

Set up a callout function
Set an offset limit for matching an unanchored pattern
Change the limit on the amount of heap used when matching
Change the backtracking match limit
Change the backtracking depth limit
Set custom memory management specifically for the match

If none of these apply, just pass NULL as the context argument of
pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match().

A match context is created, copied, and freed by the following functions:

A match context is created with default values for its parameters. These can
be changed by calling the following functions, which return 0 on success, or
PCRE2_ERROR_BADDATA if invalid data is detected.

The offset_limit parameter limits how far an unanchored search can
advance in the subject string. The default value is PCRE2_UNSET. The
pcre2_match() and pcre2_dfa_match() functions return
PCRE2_ERROR_NOMATCH if a match with a starting point before or at the given
offset is not found. The pcre2_substitute() function makes no more
substitutions.

For example, if the pattern /abc/ is matched against "123abc" with an offset
limit less than 3, the result is PCRE2_ERROR_NO_MATCH. A match can never be
found if the startoffset argument of pcre2_match(),
pcre2_dfa_match(), or pcre2_substitute() is greater than the offset
limit set in the match context.

When using this facility, you must set the PCRE2_USE_OFFSET_LIMIT option when
calling pcre2_compile() so that when JIT is in use, different code can be
compiled. If a match is started with a non-default match limit when
PCRE2_USE_OFFSET_LIMIT is not set, an error is generated.

The offset limit facility can be used to track progress when searching large
subject strings or to limit the extent of global substitutions. See also the
PCRE2_FIRSTLINE option, which requires a match to start before or at the first
newline that follows the start of matching in the subject. If this is set with
an offset limit, a match must occur in the first line and also within the
offset limit. In other words, whichever limit comes first is used.

The heap_limit parameter specifies, in units of kilobytes, the maximum
amount of heap memory that pcre2_match() may use to hold backtracking
information when running an interpretive match. This limit does not apply to
matching with the JIT optimization, which has its own memory control
arrangements (see the
pcre2jit
documentation for more details), nor does it apply to pcre2_dfa_match().
If the limit is reached, the negative error code PCRE2_ERROR_HEAPLIMIT is
returned. The default limit is set when PCRE2 is built; the default default is
very large and is essentially "unlimited".

A value for the heap limit may also be supplied by an item at the start of a
pattern of the form

(*LIMIT_HEAP=ddd)

where ddd is a decimal number. However, such a setting is ignored unless ddd is
less than the limit set by the caller of pcre2_match() or, if no such
limit is set, less than the default.

The pcre2_match() function starts out using a 20K vector on the system
stack for recording backtracking points. The more nested backtracking points
there are (that is, the deeper the search tree), the more memory is needed.
Heap memory is used only if the initial vector is too small. If the heap limit
is set to a value less than 21 (in particular, zero) no heap memory will be
used. In this case, only patterns that do not have a lot of nested backtracking
can be successfully processed.

The match_limit parameter provides a means of preventing PCRE2 from using
up too many computing resources when processing patterns that are not going to
match, but which have a very large number of possibilities in their search
trees. The classic example is a pattern that uses nested unlimited repeats.

There is an internal counter in pcre2_match() that is incremented each
time round its main matching loop. If this value reaches the match limit,
pcre2_match() returns the negative value PCRE2_ERROR_MATCHLIMIT. This has
the effect of limiting the amount of backtracking that can take place. For
patterns that are not anchored, the count restarts from zero for each position
in the subject string. This limit also applies to pcre2_dfa_match(),
though the counting is done in a different way.

When pcre2_match() is called with a pattern that was successfully
processed by pcre2_jit_compile(), the way in which matching is executed
is entirely different. However, there is still the possibility of runaway
matching that goes on for a very long time, and so the match_limit value
is also used in this case (but in a different way) to limit how long the
matching can continue.

The default value for the limit can be set when PCRE2 is built; the default
default is 10 million, which handles all but the most extreme cases. A value
for the match limit may also be supplied by an item at the start of a pattern
of the form

(*LIMIT_MATCH=ddd)

where ddd is a decimal number. However, such a setting is ignored unless ddd is
less than the limit set by the caller of pcre2_match() or
pcre2_dfa_match() or, if no such limit is set, less than the default.

This parameter limits the depth of nested backtracking in pcre2_match().
Each time a nested backtracking point is passed, a new memory "frame" is used
to remember the state of matching at that point. Thus, this parameter
indirectly limits the amount of memory that is used in a match. However,
because the size of each memory "frame" depends on the number of capturing
parentheses, the actual memory limit varies from pattern to pattern. This limit
was more useful in versions before 10.30, where function recursion was used for
backtracking.

The depth limit is not relevant, and is ignored, when matching is done using
JIT compiled code. However, it is supported by pcre2_dfa_match(), which
uses it to limit the depth of internal recursive function calls that implement
atomic groups, lookaround assertions, and pattern recursions. This is,
therefore, an indirect limit on the amount of system stack that is used. A
recursive pattern such as /(.)(?1)/, when matched to a very long string using
pcre2_dfa_match(), can use a great deal of stack.

The default value for the depth limit can be set when PCRE2 is built; the
default default is the same value as the default for the match limit. If the
limit is exceeded, pcre2_match() or pcre2_dfa_match() returns
PCRE2_ERROR_DEPTHLIMIT. A value for the depth limit may also be supplied by an
item at the start of a pattern of the form

(*LIMIT_DEPTH=ddd)

where ddd is a decimal number. However, such a setting is ignored unless ddd is
less than the limit set by the caller of pcre2_match() or
pcre2_dfa_match() or, if no such limit is set, less than the default.
CHECKING BUILD-TIME OPTIONS

int pcre2_config(uint32_t what, void *where);

The function pcre2_config() makes it possible for a PCRE2 client to
discover which optional features have been compiled into the PCRE2 library. The
pcre2build
documentation has more details about these optional features.

The first argument for pcre2_config() specifies which information is
required. The second argument is a pointer to memory into which the information
is placed. If NULL is passed, the function returns the amount of memory that is
needed for the requested information. For calls that return numerical values,
the value is in bytes; when requesting these values, where should point
to appropriately aligned memory. For calls that return strings, the required
length is given in code units, not counting the terminating zero.

When requesting information, the returned value from pcre2_config() is
non-negative on success, or the negative error code PCRE2_ERROR_BADOPTION if
the value in the first argument is not recognized. The following information is
available:

PCRE2_CONFIG_BSR

The output is a uint32_t integer whose value indicates what character
sequences the \R escape sequence matches by default. A value of
PCRE2_BSR_UNICODE means that \R matches any Unicode line ending sequence; a
value of PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, or CRLF. The
default can be overridden when a pattern is compiled.

PCRE2_CONFIG_COMPILED_WIDTHS

The output is a uint32_t integer whose lower bits indicate which code unit
widths were selected when PCRE2 was built. The 1-bit indicates 8-bit support,
and the 2-bit and 4-bit indicate 16-bit and 32-bit support, respectively.

PCRE2_CONFIG_DEPTHLIMIT

The output is a uint32_t integer that gives the default limit for the depth of
nested backtracking in pcre2_match() or the depth of nested recursions
and lookarounds in pcre2_dfa_match(). Further details are given with
pcre2_set_depth_limit() above.

PCRE2_CONFIG_HEAPLIMIT

The output is a uint32_t integer that gives, in kilobytes, the default limit
for the amount of heap memory used by pcre2_match(). Further details are
given with pcre2_set_heap_limit() above.

PCRE2_CONFIG_JIT

The output is a uint32_t integer that is set to one if support for just-in-time
compiling is available; otherwise it is set to zero.

PCRE2_CONFIG_JITTARGET

The where argument should point to a buffer that is at least 48 code
units long. (The exact length required can be found by calling
pcre2_config() with where set to NULL.) The buffer is filled with a
string that contains the name of the architecture for which the JIT compiler is
configured, for example "x86 32bit (little endian + unaligned)". If JIT support
is not available, PCRE2_ERROR_BADOPTION is returned, otherwise the number of
code units used is returned. This is the length of the string, plus one unit
for the terminating zero.

PCRE2_CONFIG_LINKSIZE

The output is a uint32_t integer that contains the number of bytes used for
internal linkage in compiled regular expressions. When PCRE2 is configured, the
value can be set to 2, 3, or 4, with the default being 2. This is the value
that is returned by pcre2_config(). However, when the 16-bit library is
compiled, a value of 3 is rounded up to 4, and when the 32-bit library is
compiled, internal linkages always use 4 bytes, so the configured value is not
relevant.

The default value of 2 for the 8-bit and 16-bit libraries is sufficient for all
but the most massive patterns, since it allows the size of the compiled pattern
to be up to 64K code units. Larger values allow larger regular expressions to
be compiled by those two libraries, but at the expense of slower matching.

PCRE2_CONFIG_MATCHLIMIT

The output is a uint32_t integer that gives the default match limit for
pcre2_match(). Further details are given with
pcre2_set_match_limit() above.

PCRE2_CONFIG_NEWLINE

The output is a uint32_t integer whose value specifies the default character
sequence that is recognized as meaning "newline". The values are:

The default should normally correspond to the standard sequence for your
operating system.

PCRE2_CONFIG_NEVER_BACKSLASH_C

The output is a uint32_t integer that is set to one if the use of \C was
permanently disabled when PCRE2 was built; otherwise it is set to zero.

PCRE2_CONFIG_PARENSLIMIT

The output is a uint32_t integer that gives the maximum depth of nesting
of parentheses (of any kind) in a pattern. This limit is imposed to cap the
amount of system stack used when a pattern is compiled. It is specified when
PCRE2 is built; the default is 250. This limit does not take into account the
stack that may already be used by the calling application. For finer control
over compilation stack usage, see pcre2_set_compile_recursion_guard().

PCRE2_CONFIG_STACKRECURSE

This parameter is obsolete and should not be used in new code. The output is a
uint32_t integer that is always set to zero.

PCRE2_CONFIG_UNICODE_VERSION

The where argument should point to a buffer that is at least 24 code
units long. (The exact length required can be found by calling
pcre2_config() with where set to NULL.) If PCRE2 has been compiled
without Unicode support, the buffer is filled with the text "Unicode not
supported". Otherwise, the Unicode version string (for example, "8.0.0") is
inserted. The number of code units used is returned. This is the length of the
string plus one unit for the terminating zero.

PCRE2_CONFIG_UNICODE

The output is a uint32_t integer that is set to one if Unicode support is
available; otherwise it is set to zero. Unicode support implies UTF support.

PCRE2_CONFIG_VERSION

The where argument should point to a buffer that is at least 24 code
units long. (The exact length required can be found by calling
pcre2_config() with where set to NULL.) The buffer is filled with
the PCRE2 version string, zero-terminated. The number of code units used is
returned. This is the length of the string plus one unit for the terminating
zero.
COMPILING A PATTERN

The pcre2_compile() function compiles a pattern into an internal form.
The pattern is defined by a pointer to a string of code units and a length (in
code units). If the pattern is zero-terminated, the length can be specified as
PCRE2_ZERO_TERMINATED. The function returns a pointer to a block of memory that
contains the compiled pattern and related data, or NULL if an error occurred.

If the compile context argument ccontext is NULL, memory for the compiled
pattern is obtained by calling malloc(). Otherwise, it is obtained from
the same memory function that was used for the compile context. The caller must
free the memory by calling pcre2_code_free() when it is no longer needed.

The function pcre2_code_copy() makes a copy of the compiled code in new
memory, using the same memory allocator as was used for the original. However,
if the code has been processed by the JIT compiler (see
below),
the JIT information cannot be copied (because it is position-dependent).
The new copy can initially be used only for non-JIT matching, though it can be
passed to pcre2_jit_compile() if required.

The pcre2_code_copy() function provides a way for individual threads in a
multithreaded application to acquire a private copy of shared compiled code.
However, it does not make a copy of the character tables used by the compiled
pattern; the new pattern code points to the same tables as the original code.
(See
"Locale Support"
below for details of these character tables.) In many applications the same
tables are used throughout, so this behaviour is appropriate. Nevertheless,
there are occasions when a copy of a compiled pattern and the relevant tables
are needed. The pcre2_code_copy_with_tables() provides this facility.
Copies of both the code and the tables are made, with the new code pointing to
the new tables. The memory for the new tables is automatically freed when
pcre2_code_free() is called for the new copy of the compiled code.

NOTE: When one of the matching functions is called, pointers to the compiled
pattern and the subject string are set in the match data block so that they can
be referenced by the substring extraction functions. After running a match, you
must not free a compiled pattern (or a subject string) until after all
operations on the
match data block
have taken place.

The options argument for pcre2_compile() contains various bit
settings that affect the compilation. It should be zero if no options are
required. The available options are described below. Some of them (in
particular, those that are compatible with Perl, but some others as well) can
also be set and unset from within the pattern (see the detailed description in
the
pcre2pattern
documentation).

For those options that can be different in different parts of the pattern, the
contents of the options argument specifies their settings at the start of
compilation. The PCRE2_ANCHORED, PCRE2_ENDANCHORED, and PCRE2_NO_UTF_CHECK
options can be set at the time of matching as well as at compile time.

Other, less frequently required compile-time parameters (for example, the
newline setting) can be provided in a compile context (as described
above).

If errorcode or erroroffset is NULL, pcre2_compile() returns
NULL immediately. Otherwise, the variables to which these point are set to an
error code and an offset (number of code units) within the pattern,
respectively, when pcre2_compile() returns NULL because a compilation
error has occurred. The values are not defined when compilation is successful
and pcre2_compile() returns a non-NULL value.

There are nearly 100 positive error codes that pcre2_compile() may return
if it finds an error in the pattern. There are also some negative error codes
that are used for invalid UTF strings. These are the same as given by
pcre2_match() and pcre2_dfa_match(), and are described in the
pcre2unicode
page. There is no separate documentation for the positive error codes, because
the textual error messages that are obtained by calling the
pcre2_get_error_message() function (see "Obtaining a textual error
message"
below)
should be self-explanatory. Macro names starting with PCRE2_ERROR_ are defined
for both positive and negative error codes in pcre2.h.

The value returned in erroroffset is an indication of where in the
pattern the error occurred. It is not necessarily the furthest point in the
pattern that was read. For example, after the error "lookbehind assertion is
not fixed length", the error offset points to the start of the failing
assertion. For an invalid UTF-8 or UTF-16 string, the offset is that of the
first code unit of the failing character.

Some errors are not detected until the whole pattern has been scanned; in these
cases, the offset passed back is the length of the pattern. Note that the
offset is in code units, not characters, even in a UTF mode. It may sometimes
point into the middle of a UTF-8 or UTF-16 character.

This code fragment shows a typical straightforward call to
pcre2_compile():

The following names for option bits are defined in the pcre2.h header
file:

PCRE2_ANCHORED

If this bit is set, the pattern is forced to be "anchored", that is, it is
constrained to match only at the first matching point in the string that is
being searched (the "subject string"). This effect can also be achieved by
appropriate constructs in the pattern itself, which is the only way to do it in
Perl.

PCRE2_ALLOW_EMPTY_CLASS

By default, for compatibility with Perl, a closing square bracket that
immediately follows an opening one is treated as a data character for the
class. When PCRE2_ALLOW_EMPTY_CLASS is set, it terminates the class, which
therefore contains no characters and so can never match.

PCRE2_ALT_BSUX

This option request alternative handling of three escape sequences, which
makes PCRE2's behaviour more like ECMAscript (aka JavaScript). When it is set:

(2) \u matches a lower case "u" character unless it is followed by four
hexadecimal digits, in which case the hexadecimal number defines the code point
to match. By default, \u causes a compile time error (Perl uses it to upper
case the following character).

(3) \x matches a lower case "x" character unless it is followed by two
hexadecimal digits, in which case the hexadecimal number defines the code point
to match. By default, as in Perl, a hexadecimal number is always expected after
\x, but it may have zero, one, or two digits (so, for example, \xz matches a
binary zero character followed by z).

PCRE2_ALT_CIRCUMFLEX

In multiline mode (when PCRE2_MULTILINE is set), the circumflex metacharacter
matches at the start of the subject (unless PCRE2_NOTBOL is set), and also
after any internal newline. However, it does not match after a newline at the
end of the subject, for compatibility with Perl. If you want a multiline
circumflex also to match after a terminating newline, you must set
PCRE2_ALT_CIRCUMFLEX.

PCRE2_ALT_VERBNAMES

By default, for compatibility with Perl, the name in any verb sequence such as
(*MARK:NAME) is any sequence of characters that does not include a closing
parenthesis. The name is not processed in any way, and it is not possible to
include a closing parenthesis in the name. However, if the PCRE2_ALT_VERBNAMES
option is set, normal backslash processing is applied to verb names and only an
unescaped closing parenthesis terminates the name. A closing parenthesis can be
included in a name either as \) or between \Q and \E. If the PCRE2_EXTENDED
or PCRE2_EXTENDED_MORE option is set, unescaped whitespace in verb names is
skipped and #-comments are recognized in this mode, exactly as in the rest of
the pattern.

PCRE2_AUTO_CALLOUT

If this bit is set, pcre2_compile() automatically inserts callout items,
all with number 255, before each pattern item, except immediately before or
after an explicit callout in the pattern. For discussion of the callout
facility, see the
pcre2callout
documentation.

PCRE2_CASELESS

If this bit is set, letters in the pattern match both upper and lower case
letters in the subject. It is equivalent to Perl's /i option, and it can be
changed within a pattern by a (?i) option setting. If PCRE2_UTF is set, Unicode
properties are used for all characters with more than one other case, and for
all characters whose code points are greater than U+007f. For lower valued
characters with only one other case, a lookup table is used for speed. When
PCRE2_UTF is not set, a lookup table is used for all code points less than 256,
and higher code points (available only in 16-bit or 32-bit mode) are treated as
not having another case.

PCRE2_DOLLAR_ENDONLY

If this bit is set, a dollar metacharacter in the pattern matches only at the
end of the subject string. Without this option, a dollar also matches
immediately before a newline at the end of the string (but not before any other
newlines). The PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is
set. There is no equivalent to this option in Perl, and no way to set it within
a pattern.

PCRE2_DOTALL

If this bit is set, a dot metacharacter in the pattern matches any character,
including one that indicates a newline. However, it only ever matches one
character, even if newlines are coded as CRLF. Without this option, a dot does
not match when the current position in the subject is at a newline. This option
is equivalent to Perl's /s option, and it can be changed within a pattern by a
(?s) option setting. A negative class such as [^a] always matches newline
characters, independent of the setting of this option.

PCRE2_DUPNAMES

If this bit is set, names used to identify capturing subpatterns need not be
unique. This can be helpful for certain types of pattern when it is known that
only one instance of the named subpattern can ever be matched. There are more
details of named subpatterns below; see also the
pcre2pattern
documentation.

PCRE2_ENDANCHORED

If this bit is set, the end of any pattern match must be right at the end of
the string being searched (the "subject string"). If the pattern match
succeeds by reaching (*ACCEPT), but does not reach the end of the subject, the
match fails at the current starting point. For unanchored patterns, a new match
is then tried at the next starting point. However, if the match succeeds by
reaching the end of the pattern, but not the end of the subject, backtracking
occurs and an alternative match may be found. Consider these two patterns:

.(*ACCEPT)|..
.|..

If matched against "abc" with PCRE2_ENDANCHORED set, the first matches "c"
whereas the second matches "bc". The effect of PCRE2_ENDANCHORED can also be
achieved by appropriate constructs in the pattern itself, which is the only way
to do it in Perl.

For DFA matching with pcre2_dfa_match(), PCRE2_ENDANCHORED applies only
to the first (that is, the longest) matched string. Other parallel matches,
which are necessarily substrings of the first one, must obviously end before
the end of the subject.

PCRE2_EXTENDED

If this bit is set, most white space characters in the pattern are totally
ignored except when escaped or inside a character class. However, white space
is not allowed within sequences such as (?> that introduce various
parenthesized subpatterns, nor within numerical quantifiers such as {1,3}.
Ignorable white space is permitted between an item and a following quantifier
and between a quantifier and a following + that indicates possessiveness.

PCRE2_EXTENDED also causes characters between an unescaped # outside a
character class and the next newline, inclusive, to be ignored, which makes it
possible to include comments inside complicated patterns. Note that the end of
this type of comment is a literal newline sequence in the pattern; escape
sequences that happen to represent a newline do not count. PCRE2_EXTENDED is
equivalent to Perl's /x option, and it can be changed within a pattern by a
(?x) option setting.

Which characters are interpreted as newlines can be specified by a setting in
the compile context that is passed to pcre2_compile() or by a special
sequence at the start of the pattern, as described in the section entitled
"Newline conventions"
in the pcre2pattern documentation. A default is defined when PCRE2 is
built.

PCRE2_EXTENDED_MORE

This option has the effect of PCRE2_EXTENDED, but, in addition, unescaped space
and horizontal tab characters are ignored inside a character class.
PCRE2_EXTENDED_MORE is equivalent to Perl's 5.26 /xx option, and it can be
changed within a pattern by a (?xx) option setting.

PCRE2_FIRSTLINE

If this option is set, the start of an unanchored pattern match must be before
or at the first newline in the subject string following the start of matching,
though the matched text may continue over the newline. If startoffset is
non-zero, the limiting newline is not necessarily the first newline in the
subject. For example, if the subject string is "abc\nxyz" (where \n
represents a single-character newline) a pattern match for "yz" succeeds with
PCRE2_FIRSTLINE if startoffset is greater than 3. See also
PCRE2_USE_OFFSET_LIMIT, which provides a more general limiting facility. If
PCRE2_FIRSTLINE is set with an offset limit, a match must occur in the first
line and also within the offset limit. In other words, whichever limit comes
first is used.

PCRE2_LITERAL

If this option is set, all meta-characters in the pattern are disabled, and it
is treated as a literal string. Matching literal strings with a regular
expression engine is not the most efficient way of doing it. If you are doing a
lot of literal matching and are worried about efficiency, you should consider
using other approaches. The only other main options that are allowed with
PCRE2_LITERAL are: PCRE2_ANCHORED, PCRE2_ENDANCHORED, PCRE2_AUTO_CALLOUT,
PCRE2_CASELESS, PCRE2_FIRSTLINE, PCRE2_NO_START_OPTIMIZE, PCRE2_NO_UTF_CHECK,
PCRE2_UTF, and PCRE2_USE_OFFSET_LIMIT. The extra options PCRE2_EXTRA_MATCH_LINE
and PCRE2_EXTRA_MATCH_WORD are also supported. Any other options cause an
error.

PCRE2_MATCH_UNSET_BACKREF

If this option is set, a back reference to an unset subpattern group matches an
empty string (by default this causes the current matching alternative to fail).
A pattern such as (\1)(a) succeeds when this option is set (assuming it can
find an "a" in the subject), whereas it fails by default, for Perl
compatibility. Setting this option makes PCRE2 behave more like ECMAscript (aka
JavaScript).

PCRE2_MULTILINE

By default, for the purposes of matching "start of line" and "end of line",
PCRE2 treats the subject string as consisting of a single line of characters,
even if it actually contains newlines. The "start of line" metacharacter (^)
matches only at the start of the string, and the "end of line" metacharacter
($) matches only at the end of the string, or before a terminating newline
(except when PCRE2_DOLLAR_ENDONLY is set). Note, however, that unless
PCRE2_DOTALL is set, the "any character" metacharacter (.) does not match at a
newline. This behaviour (for ^, $, and dot) is the same as Perl.

When PCRE2_MULTILINE it is set, the "start of line" and "end of line"
constructs match immediately following or immediately before internal newlines
in the subject string, respectively, as well as at the very start and end. This
is equivalent to Perl's /m option, and it can be changed within a pattern by a
(?m) option setting. Note that the "start of line" metacharacter does not match
after a newline at the end of the subject, for compatibility with Perl.
However, you can change this by setting the PCRE2_ALT_CIRCUMFLEX option. If
there are no newlines in a subject string, or no occurrences of ^ or $ in a
pattern, setting PCRE2_MULTILINE has no effect.

PCRE2_NEVER_BACKSLASH_C

This option locks out the use of \C in the pattern that is being compiled.
This escape can cause unpredictable behaviour in UTF-8 or UTF-16 modes, because
it may leave the current matching point in the middle of a multi-code-unit
character. This option may be useful in applications that process patterns from
external sources. Note that there is also a build-time option that permanently
locks out the use of \C.

PCRE2_NEVER_UCP

This option locks out the use of Unicode properties for handling \B, \b, \D,
\d, \S, \s, \W, \w, and some of the POSIX character classes, as described
for the PCRE2_UCP option below. In particular, it prevents the creator of the
pattern from enabling this facility by starting the pattern with (*UCP). This
option may be useful in applications that process patterns from external
sources. The option combination PCRE_UCP and PCRE_NEVER_UCP causes an error.

PCRE2_NEVER_UTF

This option locks out interpretation of the pattern as UTF-8, UTF-16, or
UTF-32, depending on which library is in use. In particular, it prevents the
creator of the pattern from switching to UTF interpretation by starting the
pattern with (*UTF). This option may be useful in applications that process
patterns from external sources. The combination of PCRE2_UTF and
PCRE2_NEVER_UTF causes an error.

PCRE2_NO_AUTO_CAPTURE

If this option is set, it disables the use of numbered capturing parentheses in
the pattern. Any opening parenthesis that is not followed by ? behaves as if it
were followed by ?: but named parentheses can still be used for capturing (and
they acquire numbers in the usual way). This is the same as Perl's /n option.
Note that, when this option is set, references to capturing groups (back
references or recursion/subroutine calls) may only refer to named groups,
though the reference can be by name or by number.

PCRE2_NO_AUTO_POSSESS

If this option is set, it disables "auto-possessification", which is an
optimization that, for example, turns a+b into a++b in order to avoid
backtracks into a+ that can never be successful. However, if callouts are in
use, auto-possessification means that some callouts are never taken. You can
set this option if you want the matching functions to do a full unoptimized
search and run all the callouts, but it is mainly provided for testing
purposes.

PCRE2_NO_DOTSTAR_ANCHOR

If this option is set, it disables an optimization that is applied when .* is
the first significant item in a top-level branch of a pattern, and all the
other branches also start with .* or with \A or \G or ^. The optimization is
automatically disabled for .* if it is inside an atomic group or a capturing
group that is the subject of a back reference, or if the pattern contains
(*PRUNE) or (*SKIP). When the optimization is not disabled, such a pattern is
automatically anchored if PCRE2_DOTALL is set for all the .* items and
PCRE2_MULTILINE is not set for any ^ items. Otherwise, the fact that any match
must start either at the start of the subject or following a newline is
remembered. Like other optimizations, this can cause callouts to be skipped.

PCRE2_NO_START_OPTIMIZE

This is an option whose main effect is at matching time. It does not change
what pcre2_compile() generates, but it does affect the output of the JIT
compiler.

There are a number of optimizations that may occur at the start of a match, in
order to speed up the process. For example, if it is known that an unanchored
match must start with a specific code unit value, the matching code searches
the subject for that value, and fails immediately if it cannot find it, without
actually running the main matching function. This means that a special item
such as (*COMMIT) at the start of a pattern is not considered until after a
suitable starting point for the match has been found. Also, when callouts or
(*MARK) items are in use, these "start-up" optimizations can cause them to be
skipped if the pattern is never actually used. The start-up optimizations are
in effect a pre-scan of the subject that takes place before the pattern is run.

The PCRE2_NO_START_OPTIMIZE option disables the start-up optimizations,
possibly causing performance to suffer, but ensuring that in cases where the
result is "no match", the callouts do occur, and that items such as (*COMMIT)
and (*MARK) are considered at every possible starting position in the subject
string.

Setting PCRE2_NO_START_OPTIMIZE may change the outcome of a matching operation.
Consider the pattern

(*COMMIT)ABC

When this is compiled, PCRE2 records the fact that a match must start with the
character "A". Suppose the subject string is "DEFABC". The start-up
optimization scans along the subject, finds "A" and runs the first match
attempt from there. The (*COMMIT) item means that the pattern must match the
current starting position, which in this case, it does. However, if the same
match is run with PCRE2_NO_START_OPTIMIZE set, the initial scan along the
subject string does not happen. The first match attempt is run starting from
"D" and when this fails, (*COMMIT) prevents any further matches being tried, so
the overall result is "no match".

There are also other start-up optimizations. For example, a minimum length for
the subject may be recorded. Consider the pattern

(*MARK:A)(X|Y)

The minimum length for a match is one character. If the subject is "ABC", there
will be attempts to match "ABC", "BC", and "C". An attempt to match an empty
string at the end of the subject does not take place, because PCRE2 knows that
the subject is now too short, and so the (*MARK) is never encountered. In this
case, the optimization does not affect the overall match result, which is still
"no match", but it does affect the auxiliary information that is returned.

PCRE2_NO_UTF_CHECK

When PCRE2_UTF is set, the validity of the pattern as a UTF string is
automatically checked. There are discussions about the validity of
UTF-8 strings,UTF-16 strings,
and
UTF-32 strings
in the
pcre2unicode
document. If an invalid UTF sequence is found, pcre2_compile() returns a
negative error code.

If you know that your pattern is a valid UTF string, and you want to skip this
check for performance reasons, you can set the PCRE2_NO_UTF_CHECK option. When
it is set, the effect of passing an invalid UTF string as a pattern is
undefined. It may cause your program to crash or loop.

Note that this option can also be passed to pcre2_match() and
pcre_dfa_match(), to suppress UTF validity checking of the subject
string.

Note also that setting PCRE2_NO_UTF_CHECK at compile time does not disable the
error that is given if an escape sequence for an invalid Unicode code point is
encountered in the pattern. In particular, the so-called "surrogate" code
points (0xd800 to 0xdfff) are invalid. If you want to allow escape sequences
such as \x{d800} you can set the PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra
option, as described in the section entitled "Extra compile options"
below.
However, this is possible only in UTF-8 and UTF-32 modes, because these values
are not representable in UTF-16.

PCRE2_UCP

This option changes the way PCRE2 processes \B, \b, \D, \d, \S, \s, \W,
\w, and some of the POSIX character classes. By default, only ASCII characters
are recognized, but if PCRE2_UCP is set, Unicode properties are used instead to
classify characters. More details are given in the section on
generic character types
in the
pcre2pattern
page. If you set PCRE2_UCP, matching one of the items it affects takes much
longer. The option is available only if PCRE2 has been compiled with Unicode
support (which is the default).

PCRE2_UNGREEDY

This option inverts the "greediness" of the quantifiers so that they are not
greedy by default, but become greedy if followed by "?". It is not compatible
with Perl. It can also be set by a (?U) option setting within the pattern.

PCRE2_USE_OFFSET_LIMIT

This option must be set for pcre2_compile() if
pcre2_set_offset_limit() is going to be used to set a non-default offset
limit in a match context for matches that use this pattern. An error is
generated if an offset limit is set without this option. For more details, see
the description of pcre2_set_offset_limit() in the
section
that describes match contexts. See also the PCRE2_FIRSTLINE
option above.

PCRE2_UTF

This option causes PCRE2 to regard both the pattern and the subject strings
that are subsequently processed as strings of UTF characters instead of
single-code-unit strings. It is available when PCRE2 is built to include
Unicode support (which is the default). If Unicode support is not available,
the use of this option provokes an error. Details of how PCRE2_UTF changes the
behaviour of PCRE2 are given in the
pcre2unicode
page.
Extra compile options

Unlike the main compile-time options, the extra options are not saved with the
compiled pattern. The option bits that can be set in a compile context by
calling the pcre2_set_compile_extra_options() function are as follows:

PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES

This option applies when compiling a pattern in UTF-8 or UTF-32 mode. It is
forbidden in UTF-16 mode, and ignored in non-UTF modes. Unicode "surrogate"
code points in the range 0xd800 to 0xdfff are used in pairs in UTF-16 to encode
code points with values in the range 0x10000 to 0x10ffff. The surrogates cannot
therefore be represented in UTF-16. They can be represented in UTF-8 and
UTF-32, but are defined as invalid code points, and cause errors if encountered
in a UTF-8 or UTF-32 string that is being checked for validity by PCRE2.

These values also cause errors if encountered in escape sequences such as
\x{d912} within a pattern. However, it seems that some applications, when
using PCRE2 to check for unwanted characters in UTF-8 strings, explicitly test
for the surrogates using escape sequences. The PCRE2_NO_UTF_CHECK option does
not disable the error that occurs, because it applies only to the testing of
input strings for UTF validity.

If the extra option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES is set, surrogate code
point values in UTF-8 and UTF-32 patterns no longer provoke errors and are
incorporated in the compiled pattern. However, they can only match subject
characters if the matching function is called with PCRE2_NO_UTF_CHECK set.

PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL

This is a dangerous option. Use with care. By default, an unrecognized escape
such as \j or a malformed one such as \x{2z} causes a compile-time error when
detected by pcre2_compile(). Perl is somewhat inconsistent in handling
such items: for example, \j is treated as a literal "j", and non-hexadecimal
digits in \x{} are just ignored, though warnings are given in both cases if
Perl's warning switch is enabled. However, a malformed octal number after \o{
always causes an error in Perl.

If the PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL extra option is passed to
pcre2_compile(), all unrecognized or erroneous escape sequences are
treated as single-character escapes. For example, \j is a literal "j" and
\x{2z} is treated as the literal string "x{2z}". Setting this option means
that typos in patterns may go undetected and have unexpected results. This is a
dangerous option. Use with care.

PCRE2_EXTRA_MATCH_LINE

This option is provided for use by the -x option of pcre2grep. It
causes the pattern only to match complete lines. This is achieved by
automatically inserting the code for "^(?:" at the start of the compiled
pattern and ")$" at the end. Thus, when PCRE2_MULTILINE is set, the matched
line may be in the middle of the subject string. This option can be used with
PCRE2_LITERAL.

PCRE2_EXTRA_MATCH_WORD

This option is provided for use by the -w option of pcre2grep. It
causes the pattern only to match strings that have a word boundary at the start
and the end. This is achieved by automatically inserting the code for "\b(?:"
at the start of the compiled pattern and ")\b" at the end. The option may be
used with PCRE2_LITERAL. However, it is ignored if PCRE2_EXTRA_MATCH_LINE is
also set.
JUST-IN-TIME (JIT) COMPILATION

These functions provide support for JIT compilation, which, if the just-in-time
compiler is available, further processes a compiled pattern into machine code
that executes much faster than the pcre2_match() interpretive matching
function. Full details are given in the
pcre2jit
documentation.

JIT compilation is a heavyweight optimization. It can take some time for
patterns to be analyzed, and for one-off matches and simple patterns the
benefit of faster execution might be offset by a much slower compilation time.
Most (but not all) patterns can be optimized by the JIT compiler.

PCRE2 handles caseless matching, and determines whether characters are letters,
digits, or whatever, by reference to a set of tables, indexed by character code
point. This applies only to characters whose code points are less than 256. By
default, higher-valued code points never match escapes such as \w or \d.
However, if PCRE2 is built with Unicode support, all characters can be tested
with \p and \P, or, alternatively, the PCRE2_UCP option can be set when a
pattern is compiled; this causes \w and friends to use Unicode property
support instead of the built-in tables.

The use of locales with Unicode is discouraged. If you are handling characters
with code points greater than 128, you should either use Unicode support, or
use locales, but not try to mix the two.

PCRE2 contains an internal set of character tables that are used by default.
These are sufficient for many applications. Normally, the internal tables
recognize only ASCII characters. However, when PCRE2 is built, it is possible
to cause the internal tables to be rebuilt in the default "C" locale of the
local system, which may cause them to be different.

The internal tables can be overridden by tables supplied by the application
that calls PCRE2. These may be created in a different locale from the default.
As more and more applications change to using Unicode, the need for this locale
support is expected to die away.

External tables are built by calling the pcre2_maketables() function, in
the relevant locale. The result can be passed to pcre2_compile() as often
as necessary, by creating a compile context and calling
pcre2_set_character_tables() to set the tables pointer therein. For
example, to build and use tables that are appropriate for the French locale
(where accented characters with values greater than 128 are treated as
letters), the following code could be used:

The locale name "fr_FR" is used on Linux and other Unix-like systems; if you
are using Windows, the name for the French locale is "french". It is the
caller's responsibility to ensure that the memory containing the tables remains
available for as long as it is needed.

The pointer that is passed (via the compile context) to pcre2_compile()
is saved with the compiled pattern, and the same tables are used by
pcre2_match() and pcre_dfa_match(). Thus, for any single pattern,
compilation and matching both happen in the same locale, but different patterns
can be processed in different locales.

The pcre2_pattern_info() function returns general information about a
compiled pattern. For information about callouts, see the
next section.
The first argument for pcre2_pattern_info() is a pointer to the compiled
pattern. The second argument specifies which piece of information is required,
and the third argument is a pointer to a variable to receive the data. If the
third argument is NULL, the first argument is ignored, and the function returns
the size in bytes of the variable that is required for the information
requested. Otherwise, the yield of the function is zero for success, or one of
the following negative numbers:

PCRE2_ERROR_NULL the argument code was NULL
PCRE2_ERROR_BADMAGIC the "magic number" was not found
PCRE2_ERROR_BADOPTION the value of what was invalid
PCRE2_ERROR_UNSET the requested field is not set

The "magic number" is placed at the start of each compiled pattern as an simple
check against passing an arbitrary memory pointer. Here is a typical call of
pcre2_pattern_info(), to obtain the length of the compiled pattern:

The possible values for the second argument are defined in pcre2.h, and
are as follows:

PCRE2_INFO_ALLOPTIONS
PCRE2_INFO_ARGOPTIONS
PCRE2_INFO_EXTRAOPTIONS

Return copies of the pattern's options. The third argument should point to a
uint32_t variable. PCRE2_INFO_ARGOPTIONS returns exactly the options that
were passed to pcre2_compile(), whereas PCRE2_INFO_ALLOPTIONS returns
the compile options as modified by any top-level (*XXX) option settings such as
(*UTF) at the start of the pattern itself. PCRE2_INFO_EXTRAOPTIONS returns the
extra options that were set in the compile context by calling the
pcre2_set_compile_extra_options() function.

For example, if the pattern /(*UTF)abc/ is compiled with the PCRE2_EXTENDED
option, the result for PCRE2_INFO_ALLOPTIONS is PCRE2_EXTENDED and PCRE2_UTF.
Option settings such as (?i) that can change within a pattern do not affect the
result of PCRE2_INFO_ALLOPTIONS, even if they appear right at the start of the
pattern. (This was different in some earlier releases.)

A pattern compiled without PCRE2_ANCHORED is automatically anchored by PCRE2 if
the first significant item in every top-level branch is one of the following:

When .* is the first significant item, anchoring is possible only when all the
following are true:

.* is not in an atomic group
.* is not in a capturing group that is the subject of a back reference
PCRE2_DOTALL is in force for .*
Neither (*PRUNE) nor (*SKIP) appears in the pattern
PCRE2_NO_DOTSTAR_ANCHOR is not set

For patterns that are auto-anchored, the PCRE2_ANCHORED bit is set in the
options returned for PCRE2_INFO_ALLOPTIONS.

PCRE2_INFO_BACKREFMAX

Return the number of the highest back reference in the pattern. The third
argument should point to an uint32_t variable. Named subpatterns acquire
numbers as well as names, and these count towards the highest back reference.
Back references such as \4 or \g{12} match the captured characters of the
given group, but in addition, the check that a capturing group is set in a
conditional subpattern such as (?(3)a|b) is also a back reference. Zero is
returned if there are no back references.

PCRE2_INFO_BSR

The output is a uint32_t whose value indicates what character sequences the \R
escape sequence matches. A value of PCRE2_BSR_UNICODE means that \R matches
any Unicode line ending sequence; a value of PCRE2_BSR_ANYCRLF means that \R
matches only CR, LF, or CRLF.

PCRE2_INFO_CAPTURECOUNT

Return the highest capturing subpattern number in the pattern. In patterns
where (?| is not used, this is also the total number of capturing subpatterns.
The third argument should point to an uint32_t variable.

PCRE2_INFO_DEPTHLIMIT

If the pattern set a backtracking depth limit by including an item of the form
(*LIMIT_DEPTH=nnnn) at the start, the value is returned. The third argument
should point to an unsigned 32-bit integer. If no such value has been set, the
call to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET. Note
that this limit will only be used during matching if it is less than the limit
set or defaulted by the caller of the match function.

PCRE2_INFO_FIRSTBITMAP

In the absence of a single first code unit for a non-anchored pattern,
pcre2_compile() may construct a 256-bit table that defines a fixed set of
values for the first code unit in any match. For example, a pattern that starts
with [abc] results in a table with three bits set. When code unit values
greater than 255 are supported, the flag bit for 255 means "any code unit of
value 255 or above". If such a table was constructed, a pointer to it is
returned. Otherwise NULL is returned. The third argument should point to an
const uint8_t * variable.

PCRE2_INFO_FIRSTCODETYPE

Return information about the first code unit of any matched string, for a
non-anchored pattern. The third argument should point to an uint32_t
variable. If there is a fixed first value, for example, the letter "c" from a
pattern such as (cat|cow|coyote), 1 is returned, and the value can be retrieved
using PCRE2_INFO_FIRSTCODEUNIT. If there is no fixed first value, but it is
known that a match can occur only at the start of the subject or following a
newline in the subject, 2 is returned. Otherwise, and for anchored patterns, 0
is returned.

PCRE2_INFO_FIRSTCODEUNIT

Return the value of the first code unit of any matched string for a pattern
where PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise return 0. The third
argument should point to an uint32_t variable. In the 8-bit library, the
value is always less than 256. In the 16-bit library the value can be up to
0xffff. In the 32-bit library in UTF-32 mode the value can be up to 0x10ffff,
and up to 0xffffffff when not using UTF-32 mode.

PCRE2_INFO_FRAMESIZE

Return the size (in bytes) of the data frames that are used to remember
backtracking positions when the pattern is processed by pcre2_match()
without the use of JIT. The third argument should point to an size_t
variable. The frame size depends on the number of capturing parentheses in the
pattern. Each additional capturing group adds two PCRE2_SIZE variables.

PCRE2_INFO_HASBACKSLASHC

Return 1 if the pattern contains any instances of \C, otherwise 0. The third
argument should point to an uint32_t variable.

PCRE2_INFO_HASCRORLF

Return 1 if the pattern contains any explicit matches for CR or LF characters,
otherwise 0. The third argument should point to an uint32_t variable. An
explicit match is either a literal CR or LF character, or \r or \n or one of
the equivalent hexadecimal or octal escape sequences.

PCRE2_INFO_HEAPLIMIT

If the pattern set a heap memory limit by including an item of the form
(*LIMIT_HEAP=nnnn) at the start, the value is returned. The third argument
should point to an unsigned 32-bit integer. If no such value has been set, the
call to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET. Note
that this limit will only be used during matching if it is less than the limit
set or defaulted by the caller of the match function.

PCRE2_INFO_JCHANGED

Return 1 if the (?J) or (?-J) option setting is used in the pattern, otherwise
0. The third argument should point to an uint32_t variable. (?J) and
(?-J) set and unset the local PCRE2_DUPNAMES option, respectively.

PCRE2_INFO_JITSIZE

If the compiled pattern was successfully processed by
pcre2_jit_compile(), return the size of the JIT compiled code, otherwise
return zero. The third argument should point to a size_t variable.

PCRE2_INFO_LASTCODETYPE

Returns 1 if there is a rightmost literal code unit that must exist in any
matched string, other than at its start. The third argument should point to an
uint32_t variable. If there is no such value, 0 is returned. When 1 is
returned, the code unit value itself can be retrieved using
PCRE2_INFO_LASTCODEUNIT. For anchored patterns, a last literal value is
recorded only if it follows something of variable length. For example, for the
pattern /^a\d+z\d+/ the returned value is 1 (with "z" returned from
PCRE2_INFO_LASTCODEUNIT), but for /^a\dz\d/ the returned value is 0.

PCRE2_INFO_LASTCODEUNIT

Return the value of the rightmost literal code unit that must exist in any
matched string, other than at its start, for a pattern where
PCRE2_INFO_LASTCODETYPE returns 1. Otherwise, return 0. The third argument
should point to an uint32_t variable.

PCRE2_INFO_MATCHEMPTY

Return 1 if the pattern might match an empty string, otherwise 0. The third
argument should point to an uint32_t variable. When a pattern contains
recursive subroutine calls it is not always possible to determine whether or
not it can match an empty string. PCRE2 takes a cautious approach and returns 1
in such cases.

PCRE2_INFO_MATCHLIMIT

If the pattern set a match limit by including an item of the form
(*LIMIT_MATCH=nnnn) at the start, the value is returned. The third argument
should point to an unsigned 32-bit integer. If no such value has been set, the
call to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET. Note
that this limit will only be used during matching if it is less than the limit
set or defaulted by the caller of the match function.

PCRE2_INFO_MAXLOOKBEHIND

Return the number of characters (not code units) in the longest lookbehind
assertion in the pattern. The third argument should point to an unsigned 32-bit
integer. This information is useful when doing multi-segment matching using the
partial matching facilities. Note that the simple assertions \b and \B
require a one-character lookbehind. \A also registers a one-character
lookbehind, though it does not actually inspect the previous character. This is
to ensure that at least one character from the old segment is retained when a
new segment is processed. Otherwise, if there are no lookbehinds in the
pattern, \A might match incorrectly at the start of a second or subsequent
segment.

PCRE2_INFO_MINLENGTH

If a minimum length for matching subject strings was computed, its value is
returned. Otherwise the returned value is 0. The value is a number of
characters, which in UTF mode may be different from the number of code units.
The third argument should point to an uint32_t variable. The value is a
lower bound to the length of any matching string. There may not be any strings
of that length that do actually match, but every string that does match is at
least that long.

PCRE2_INFO_NAMECOUNT
PCRE2_INFO_NAMEENTRYSIZE
PCRE2_INFO_NAMETABLE

PCRE2 supports the use of named as well as numbered capturing parentheses. The
names are just an additional way of identifying the parentheses, which still
acquire numbers. Several convenience functions such as
pcre2_substring_get_byname() are provided for extracting captured
substrings by name. It is also possible to extract the data directly, by first
converting the name to a number in order to access the correct pointers in the
output vector (described with pcre2_match() below). To do the conversion,
you need to use the name-to-number map, which is described by these three
values.

The map consists of a number of fixed-size entries. PCRE2_INFO_NAMECOUNT gives
the number of entries, and PCRE2_INFO_NAMEENTRYSIZE gives the size of each
entry in code units; both of these return a uint32_t value. The entry
size depends on the length of the longest name.

PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table. This is
a PCRE2_SPTR pointer to a block of code units. In the 8-bit library, the first
two bytes of each entry are the number of the capturing parenthesis, most
significant byte first. In the 16-bit library, the pointer points to 16-bit
code units, the first of which contains the parenthesis number. In the 32-bit
library, the pointer points to 32-bit code units, the first of which contains
the parenthesis number. The rest of the entry is the corresponding name, zero
terminated.

The names are in alphabetical order. If (?| is used to create multiple groups
with the same number, as described in the
section on duplicate subpattern numbers
in the
pcre2pattern
page, the groups may be given the same name, but there is only one entry in the
table. Different names for groups of the same number are not permitted.

Duplicate names for subpatterns with different numbers are permitted, but only
if PCRE2_DUPNAMES is set. They appear in the table in the order in which they
were found in the pattern. In the absence of (?| this is the order of
increasing number; when (?| is used this is not necessarily the case because
later subpatterns may have lower numbers.

As a simple example of the name/number table, consider the following pattern
after compilation by the 8-bit library (assume PCRE2_EXTENDED is set, so white
space - including newlines - is ignored):

(?<date> (?<year>(\d\d)?\d\d) - (?<month>\d\d) - (?<day>\d\d) )

There are four named subpatterns, so the table has four entries, and each entry
in the table is eight bytes long. The table is as follows, with non-printing
bytes shows in hexadecimal, and undefined bytes shown as ??:

This identifies the character sequence that will be recognized as meaning
"newline" while matching.

PCRE2_INFO_SIZE

Return the size of the compiled pattern in bytes (for all three libraries). The
third argument should point to a size_t variable. This value includes the
size of the general data block that precedes the code units of the compiled
pattern itself. The value that is used when pcre2_compile() is getting
memory in which to place the compiled pattern may be slightly larger than the
value returned by this option, because there are cases where the code that
calculates the size has to over-estimate. Processing a pattern with the JIT
compiler does not alter the value returned by this option.
INFORMATION ABOUT A PATTERN'S CALLOUTS

A script language that supports the use of string arguments in callouts might
like to scan all the callouts in a pattern before running the match. This can
be done by calling pcre2_callout_enumerate(). The first argument is a
pointer to a compiled pattern, the second points to a callback function, and
the third is arbitrary user data. The callback function is called for every
callout in the pattern in the order in which they appear. Its first argument is
a pointer to a callout enumeration block, and its second argument is the
user_data value that was passed to pcre2_callout_enumerate(). The
contents of the callout enumeration block are described in the
pcre2callout
documentation, which also gives further details about callouts.

It is possible to save compiled patterns on disc or elsewhere, and reload them
later, subject to a number of restrictions. The functions whose names begin
with pcre2_serialize_ are used for this purpose. They are described in
the
pcre2serialize
documentation.

Information about a successful or unsuccessful match is placed in a match
data block, which is an opaque structure that is accessed by function calls. In
particular, the match data block contains a vector of offsets into the subject
string that define the matched part of the subject and any substrings that were
captured. This is known as the ovector.

Before calling pcre2_match(), pcre2_dfa_match(), or
pcre2_jit_match() you must create a match data block by calling one of
the creation functions above. For pcre2_match_data_create(), the first
argument is the number of pairs of offsets in the ovector. One pair of
offsets is required to identify the string that matched the whole pattern, with
an additional pair for each captured substring. For example, a value of 4
creates enough space to record the matched portion of the subject plus three
captured substrings. A minimum of at least 1 pair is imposed by
pcre2_match_data_create(), so it is always possible to return the overall
matched string.

The second argument of pcre2_match_data_create() is a pointer to a
general context, which can specify custom memory management for obtaining the
memory for the match data block. If you are not using custom memory management,
pass NULL, which causes malloc() to be used.

For pcre2_match_data_create_from_pattern(), the first argument is a
pointer to a compiled pattern. The ovector is created to be exactly the right
size to hold all the substrings a pattern might capture. The second argument is
again a pointer to a general context, but in this case if NULL is passed, the
memory is obtained using the same allocator that was used for the compiled
pattern (custom or default).

A match data block can be used many times, with the same or different compiled
patterns. You can extract information from a match data block after a match
operation has finished, using functions that are described in the sections on
matched strings
and
other match data
below.

When a call of pcre2_match() fails, valid data is available in the match
block only when the error is PCRE2_ERROR_NOMATCH, PCRE2_ERROR_PARTIAL, or one
of the error codes for an invalid UTF string. Exactly what is available depends
on the error, and is detailed below.

When one of the matching functions is called, pointers to the compiled pattern
and the subject string are set in the match data block so that they can be
referenced by the extraction functions. After running a match, you must not
free a compiled pattern or a subject string until after all operations on the
match data block (for that match) have taken place.

When a match data block itself is no longer needed, it should be freed by
calling pcre2_match_data_free().

The function pcre2_match() is called to match a subject string against a
compiled pattern, which is passed in the code argument. You can call
pcre2_match() with the same code argument as many times as you
like, in order to find multiple matches in the subject string or to match
different subject strings with the same pattern.

This function is the main matching facility of the library, and it operates in
a Perl-like manner. For specialist use there is also an alternative matching
function, which is described
below
in the section about the pcre2_dfa_match() function.

If the subject string is zero-terminated, the length can be given as
PCRE2_ZERO_TERMINATED. A match context must be provided if certain less common
matching parameters are to be changed. For details, see the section on
the match context
above.
The string to be matched by pcre2_match()

The subject string is passed to pcre2_match() as a pointer in
subject, a length in length, and a starting offset in
startoffset. The length and offset are in code units, not characters.
That is, they are in bytes for the 8-bit library, 16-bit code units for the
16-bit library, and 32-bit code units for the 32-bit library, whether or not
UTF processing is enabled.

If startoffset is greater than the length of the subject,
pcre2_match() returns PCRE2_ERROR_BADOFFSET. When the starting offset is
zero, the search for a match starts at the beginning of the subject, and this
is by far the most common case. In UTF-8 or UTF-16 mode, the starting offset
must point to the start of a character, or to the end of the subject (in UTF-32
mode, one code unit equals one character, so all offsets are valid). Like the
pattern string, the subject may contain binary zeroes.

A non-zero starting offset is useful when searching for another match in the
same subject by calling pcre2_match() again after a previous success.
Setting startoffset differs from passing over a shortened string and
setting PCRE2_NOTBOL in the case of a pattern that begins with any kind of
lookbehind. For example, consider the pattern

\Biss\B

which finds occurrences of "iss" in the middle of words. (\B matches only if
the current position in the subject is not a word boundary.) When applied to
the string "Mississipi" the first call to pcre2_match() finds the first
occurrence. If pcre2_match() is called again with just the remainder of
the subject, namely "issipi", it does not match, because \B is always false at
the start of the subject, which is deemed to be a word boundary. However, if
pcre2_match() is passed the entire string again, but with
startoffset set to 4, it finds the second occurrence of "iss" because it
is able to look behind the starting point to discover that it is preceded by a
letter.

Finding all the matches in a subject is tricky when the pattern can match an
empty string. It is possible to emulate Perl's /g behaviour by first trying the
match again at the same offset, with the PCRE2_NOTEMPTY_ATSTART and
PCRE2_ANCHORED options, and then if that fails, advancing the starting offset
and trying an ordinary match again. There is some code that demonstrates how to
do this in the
pcre2demo
sample program. In the most general case, you have to check to see if the
newline convention recognizes CRLF as a newline, and if so, and the current
character is CR followed by LF, advance the starting offset by two characters
instead of one.

If a non-zero starting offset is passed when the pattern is anchored, a single
attempt to match at the given offset is made. This can only succeed if the
pattern does not require the match to be at the start of the subject. In other
words, the anchoring must be the result of setting the PCRE2_ANCHORED option or
the use of .* with PCRE2_DOTALL, not by starting the pattern with ^ or \A.

Option bits for pcre2_match()

The unused bits of the options argument for pcre2_match() must be
zero. The only bits that may be set are PCRE2_ANCHORED, PCRE2_ENDANCHORED,
PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
PCRE2_NO_JIT, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT.
Their action is described below.

Setting PCRE2_ANCHORED or PCRE2_ENDANCHORED at match time is not supported by
the just-in-time (JIT) compiler. If it is set, JIT matching is disabled and the
interpretive code in pcre2_match() is run. Apart from PCRE2_NO_JIT
(obviously), the remaining options are supported for JIT matching.

PCRE2_ANCHORED

The PCRE2_ANCHORED option limits pcre2_match() to matching at the first
matching position. If a pattern was compiled with PCRE2_ANCHORED, or turned out
to be anchored by virtue of its contents, it cannot be made unachored at
matching time. Note that setting the option at match time disables JIT
matching.

PCRE2_ENDANCHORED

If the PCRE2_ENDANCHORED option is set, any string that pcre2_match()
matches must be right at the end of the subject string. Note that setting the
option at match time disables JIT matching.

PCRE2_NOTBOL

This option specifies that first character of the subject string is not the
beginning of a line, so the circumflex metacharacter should not match before
it. Setting this without having set PCRE2_MULTILINE at compile time causes
circumflex never to match. This option affects only the behaviour of the
circumflex metacharacter. It does not affect \A.

PCRE2_NOTEOL

This option specifies that the end of the subject string is not the end of a
line, so the dollar metacharacter should not match it nor (except in multiline
mode) a newline immediately before it. Setting this without having set
PCRE2_MULTILINE at compile time causes dollar never to match. This option
affects only the behaviour of the dollar metacharacter. It does not affect \Z
or \z.

PCRE2_NOTEMPTY

An empty string is not considered to be a valid match if this option is set. If
there are alternatives in the pattern, they are tried. If all the alternatives
match the empty string, the entire match fails. For example, if the pattern

a?b?

is applied to a string not beginning with "a" or "b", it matches an empty
string at the start of the subject. With PCRE2_NOTEMPTY set, this match is not
valid, so pcre2_match() searches further into the string for occurrences
of "a" or "b".

PCRE2_NOTEMPTY_ATSTART

This is like PCRE2_NOTEMPTY, except that it locks out an empty string match
only at the first matching position, that is, at the start of the subject plus
the starting offset. An empty string match later in the subject is permitted.
If the pattern is anchored, such a match can occur only if the pattern contains
\K.

PCRE2_NO_JIT

By default, if a pattern has been successfully processed by
pcre2_jit_compile(), JIT is automatically used when pcre2_match()
is called with options that JIT supports. Setting PCRE2_NO_JIT disables the use
of JIT; it forces matching to be done by the interpreter.

PCRE2_NO_UTF_CHECK

When PCRE2_UTF is set at compile time, the validity of the subject as a UTF
string is checked by default when pcre2_match() is subsequently called.
If a non-zero starting offset is given, the check is applied only to that part
of the subject that could be inspected during matching, and there is a check
that the starting offset points to the first code unit of a character or to the
end of the subject. If there are no lookbehind assertions in the pattern, the
check starts at the starting offset. Otherwise, it starts at the length of the
longest lookbehind before the starting offset, or at the start of the subject
if there are not that many characters before the starting offset. Note that the
sequences \b and \B are one-character lookbehinds.

The check is carried out before any other processing takes place, and a
negative error code is returned if the check fails. There are several UTF error
codes for each code unit width, corresponding to different problems with the
code unit sequence. There are discussions about the validity of
UTF-8 strings,UTF-16 strings,
and
UTF-32 strings
in the
pcre2unicode
page.

If you know that your subject is valid, and you want to skip these checks for
performance reasons, you can set the PCRE2_NO_UTF_CHECK option when calling
pcre2_match(). You might want to do this for the second and subsequent
calls to pcre2_match() if you are making repeated calls to find other
matches in the same subject string.

WARNING: When PCRE2_NO_UTF_CHECK is set, the effect of passing an invalid
string as a subject, or an invalid value of startoffset, is undefined.
Your program may crash or loop indefinitely.

PCRE2_PARTIAL_HARD
PCRE2_PARTIAL_SOFT

These options turn on the partial matching feature. A partial match occurs if
the end of the subject string is reached successfully, but there are not enough
subject characters to complete the match. If this happens when
PCRE2_PARTIAL_SOFT (but not PCRE2_PARTIAL_HARD) is set, matching continues by
testing any remaining alternatives. Only if no complete match can be found is
PCRE2_ERROR_PARTIAL returned instead of PCRE2_ERROR_NOMATCH. In other words,
PCRE2_PARTIAL_SOFT specifies that the caller is prepared to handle a partial
match, but only if no complete match can be found.

If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In this case, if
a partial match is found, pcre2_match() immediately returns
PCRE2_ERROR_PARTIAL, without considering any other alternatives. In other
words, when PCRE2_PARTIAL_HARD is set, a partial match is considered to be more
important that an alternative complete match.

There is a more detailed discussion of partial and multi-segment matching, with
examples, in the
pcre2partial
documentation.

When PCRE2 is built, a default newline convention is set; this is usually the
standard convention for the operating system. The default can be overridden in
a
compile context
by calling pcre2_set_newline(). It can also be overridden by starting a
pattern string with, for example, (*CRLF), as described in the
section on newline conventions
in the
pcre2pattern
page. During matching, the newline choice affects the behaviour of the dot,
circumflex, and dollar metacharacters. It may also alter the way the match
starting position is advanced after a match failure for an unanchored pattern.

When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is set as
the newline convention, and a match attempt for an unanchored pattern fails
when the current starting position is at a CRLF sequence, and the pattern
contains no explicit matches for CR or LF characters, the match position is
advanced by two characters instead of one, in other words, to after the CRLF.

The above rule is a compromise that makes the most common cases work as
expected. For example, if the pattern is .+A (and the PCRE2_DOTALL option is
not set), it does not match the string "\r\nA" because, after failing at the
start, it skips both the CR and the LF before retrying. However, the pattern
[\r\n]A does match that string, because it contains an explicit CR or LF
reference, and so advances only by one character after the first failure.

An explicit match for CR of LF is either a literal appearance of one of those
characters in the pattern, or one of the \r or \n or equivalent octal or
hexadecimal escape sequences. Implicit matches such as [^X] do not count, nor
does \s, even though it includes CR and LF in the characters that it matches.

Notwithstanding the above, anomalous effects may still occur when CRLF is a
valid newline sequence and explicit \r or \n escapes appear in the pattern.

In general, a pattern matches a certain portion of the subject, and in
addition, further substrings from the subject may be picked out by
parenthesized parts of the pattern. Following the usage in Jeffrey Friedl's
book, this is called "capturing" in what follows, and the phrase "capturing
subpattern" or "capturing group" is used for a fragment of a pattern that picks
out a substring. PCRE2 supports several other kinds of parenthesized subpattern
that do not cause substrings to be captured. The pcre2_pattern_info()
function can be used to find out how many capturing subpatterns there are in a
compiled pattern.

You can use auxiliary functions for accessing captured substrings
by number
or
by name,
as described in sections below.

Alternatively, you can make direct use of the vector of PCRE2_SIZE values,
called the ovector, which contains the offsets of captured strings. It is
part of the
match data block.
The function pcre2_get_ovector_pointer() returns the address of the
ovector, and pcre2_get_ovector_count() returns the number of pairs of
values it contains.

Within the ovector, the first in each pair of values is set to the offset of
the first code unit of a substring, and the second is set to the offset of the
first code unit after the end of a substring. These values are always code unit
offsets, not character offsets. That is, they are byte offsets in the 8-bit
library, 16-bit offsets in the 16-bit library, and 32-bit offsets in the 32-bit
library.

After a partial match (error return PCRE2_ERROR_PARTIAL), only the first pair
of offsets (that is, ovector[0] and ovector[1]) are set. They
identify the part of the subject that was partially matched. See the
pcre2partial
documentation for details of partial matching.

After a fully successful match, the first pair of offsets identifies the
portion of the subject string that was matched by the entire pattern. The next
pair is used for the first captured substring, and so on. The value returned by
pcre2_match() is one more than the highest numbered pair that has been
set. For example, if two substrings have been captured, the returned value is
3. If there are no captured substrings, the return value from a successful
match is 1, indicating that just the first pair of offsets has been set.

If a pattern uses the \K escape sequence within a positive assertion, the
reported start of a successful match can be greater than the end of the match.
For example, if the pattern (?=ab\K) is matched against "ab", the start and
end offset values for the match are 2 and 0.

If a capturing subpattern group is matched repeatedly within a single match
operation, it is the last portion of the subject that it matched that is
returned.

If the ovector is too small to hold all the captured substring offsets, as much
as possible is filled in, and the function returns a value of zero. If captured
substrings are not of interest, pcre2_match() may be called with a match
data block whose ovector is of minimum length (that is, one pair).

It is possible for capturing subpattern number n+1 to match some part of
the subject when subpattern n has not been used at all. For example, if
the string "abc" is matched against the pattern (a|(z))(bc) the return from the
function is 4, and subpatterns 1 and 3 are matched, but 2 is not. When this
happens, both values in the offset pairs corresponding to unused subpatterns
are set to PCRE2_UNSET.

Offset values that correspond to unused subpatterns at the end of the
expression are also set to PCRE2_UNSET. For example, if the string "abc" is
matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not matched.
The return from the function is 2, because the highest used capturing
subpattern number is 1. The offsets for for the second and third capturing
subpatterns (assuming the vector is large enough, of course) are set to
PCRE2_UNSET.

Elements in the ovector that do not correspond to capturing parentheses in the
pattern are never changed. That is, if a pattern contains n capturing
parentheses, no more than ovector[0] to ovector[2n+1] are set by
pcre2_match(). The other elements retain whatever values they previously
had.

As well as the offsets in the ovector, other information about a match is
retained in the match data block and can be retrieved by the above functions in
appropriate circumstances. If they are called at other times, the result is
undefined.

After a successful match, a partial match (PCRE2_ERROR_PARTIAL), or a failure
to match (PCRE2_ERROR_NOMATCH), a (*MARK), (*PRUNE), or (*THEN) name may be
available. The function pcre2_get_mark() can be called to access this
name. The same function applies to all three verbs. It returns a pointer to the
zero-terminated name, which is within the compiled pattern. If no name is
available, NULL is returned. The length of the name (excluding the terminating
zero) is stored in the code unit that precedes the name. You should use this
length instead of relying on the terminating zero if the name might contain a
binary zero.

After a successful match, the name that is returned is the last (*MARK),
(*PRUNE), or (*THEN) name encountered on the matching path through the pattern.
Instances of (*PRUNE) and (*THEN) without names are ignored. Thus, for example,
if the matching path contains (*MARK:A)(*PRUNE), the name "A" is returned.
After a "no match" or a partial match, the last encountered name is returned.
For example, consider this pattern:

^(*MARK:A)((*MARK:B)a|b)c

When it matches "bc", the returned name is A. The B mark is "seen" in the first
branch of the group, but it is not on the matching path. On the other hand,
when this pattern fails to match "bx", the returned name is B.

After a successful match, a partial match, or one of the invalid UTF errors
(for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can be
called. After a successful or partial match it returns the code unit offset of
the character at which the match started. For a non-partial match, this can be
different to the value of ovector[0] if the pattern contains the \K
escape sequence. After a partial match, however, this value is always the same
as ovector[0] because \K does not affect the result of a partial match.

After a UTF check failure, pcre2_get_startchar() can be used to obtain
the code unit offset of the invalid UTF character. Details are given in the
pcre2unicode
page.

If pcre2_match() fails, it returns a negative number. This can be
converted to a text string by calling the pcre2_get_error_message()
function (see "Obtaining a textual error message"
below).
Negative error codes are also returned by other functions, and are documented
with them. The codes are given names in the header file. If UTF checking is in
force and an invalid UTF subject string is detected, one of a number of
UTF-specific negative error codes is returned. Details are given in the
pcre2unicode
page. The following are the other errors that may be returned by
pcre2_match():

PCRE2_ERROR_NOMATCH

The subject string did not match the pattern.

PCRE2_ERROR_PARTIAL

The subject string did not match, but it did match partially. See the
pcre2partial
documentation for details of partial matching.

PCRE2_ERROR_BADMAGIC

PCRE2 stores a 4-byte "magic number" at the start of the compiled code, to
catch the case when it is passed a junk pointer. This is the error that is
returned when the magic number is not present.

PCRE2_ERROR_BADMODE

This error is given when a compiled pattern is passed to a function in a
library of a different code unit width, for example, a pattern compiled by
the 8-bit library is passed to a 16-bit or 32-bit library function.

PCRE2_ERROR_BADOFFSET

The value of startoffset was greater than the length of the subject.

PCRE2_ERROR_BADOPTION

An unrecognized bit was set in the options argument.

PCRE2_ERROR_BADUTFOFFSET

The UTF code unit sequence that was passed as a subject was checked and found
to be valid (the PCRE2_NO_UTF_CHECK option was not set), but the value of
startoffset did not point to the beginning of a UTF character or the end
of the subject.

PCRE2_ERROR_CALLOUT

This error is never generated by pcre2_match() itself. It is provided for
use by callout functions that want to cause pcre2_match() or
pcre2_callout_enumerate() to return a distinctive error code. See the
pcre2callout
documentation for details.

PCRE2_ERROR_DEPTHLIMIT

The nested backtracking depth limit was reached.

PCRE2_ERROR_HEAPLIMIT

The heap limit was reached.

PCRE2_ERROR_INTERNAL

An unexpected internal error has occurred. This error could be caused by a bug
in PCRE2 or by overwriting of the compiled pattern.

PCRE2_ERROR_JIT_STACKLIMIT

This error is returned when a pattern that was successfully studied using JIT
is being matched, but the memory available for the just-in-time processing
stack is not large enough. See the
pcre2jit
documentation for more details.

PCRE2_ERROR_MATCHLIMIT

The backtracking match limit was reached.

PCRE2_ERROR_NOMEMORY

If a pattern contains many nested backtracking points, heap memory is used to
remember them. This error is given when the memory allocation function (default
or custom) fails. Note that a different error, PCRE2_ERROR_HEAPLIMIT, is given
if the amount of memory needed exceeds the heap limit.

PCRE2_ERROR_NULL

Either the code, subject, or match_data argument was passed
as NULL.

PCRE2_ERROR_RECURSELOOP

This error is returned when pcre2_match() detects a recursion loop within
the pattern. Specifically, it means that either the whole pattern or a
subpattern has been called recursively for the second time at the same position
in the subject string. Some simple patterns that might do this are detected and
faulted at compile time, but more complicated cases, in particular mutual
recursions between two different subpatterns, cannot be detected until matching
is attempted.
OBTAINING A TEXTUAL ERROR MESSAGE

A text message for an error code from any PCRE2 function (compile, match, or
auxiliary) can be obtained by calling pcre2_get_error_message(). The code
is passed as the first argument, with the remaining two arguments specifying a
code unit buffer and its length in code units, into which the text message is
placed. The message is returned in code units of the appropriate width for the
library that is being used.

The returned message is terminated with a trailing zero, and the function
returns the number of code units used, excluding the trailing zero. If the
error number is unknown, the negative error code PCRE2_ERROR_BADDATA is
returned. If the buffer is too small, the message is truncated (but still with
a trailing zero), and the negative error code PCRE2_ERROR_NOMEMORY is returned.
None of the messages are very long; a buffer size of 120 code units is ample.

Captured substrings can be accessed directly by using the ovector as described
above.
For convenience, auxiliary functions are provided for extracting captured
substrings as new, separate, zero-terminated strings. A substring that contains
a binary zero is correctly extracted and has a further zero added on the end,
but the result is not, of course, a C string.

The functions in this section identify substrings by number. The number zero
refers to the entire matched substring, with higher numbers referring to
substrings captured by parenthesized groups. After a partial match, only
substring zero is available. An attempt to extract any other substring gives
the error PCRE2_ERROR_PARTIAL. The next section describes similar functions for
extracting captured substrings by name.

If a pattern uses the \K escape sequence within a positive assertion, the
reported start of a successful match can be greater than the end of the match.
For example, if the pattern (?=ab\K) is matched against "ab", the start and
end offset values for the match are 2 and 0. In this situation, calling these
functions with a zero substring number extracts a zero-length empty string.

You can find the length in code units of a captured substring without
extracting it by calling pcre2_substring_length_bynumber(). The first
argument is a pointer to the match data block, the second is the group number,
and the third is a pointer to a variable into which the length is placed. If
you just want to know whether or not the substring has been captured, you can
pass the third argument as NULL.

The pcre2_substring_copy_bynumber() function copies a captured substring
into a supplied buffer, whereas pcre2_substring_get_bynumber() copies it
into new memory, obtained using the same memory allocation function that was
used for the match data block. The first two arguments of these functions are a
pointer to the match data block and a capturing group number.

The final arguments of pcre2_substring_copy_bynumber() are a pointer to
the buffer and a pointer to a variable that contains its length in code units.
This is updated to contain the actual number of code units used for the
extracted substring, excluding the terminating zero.

For pcre2_substring_get_bynumber() the third and fourth arguments point
to variables that are updated with a pointer to the new memory and the number
of code units that comprise the substring, again excluding the terminating
zero. When the substring is no longer needed, the memory should be freed by
calling pcre2_substring_free().

The return value from all these functions is zero for success, or a negative
error code. If the pattern match failed, the match failure code is returned.
If a substring number greater than zero is used after a partial match,
PCRE2_ERROR_PARTIAL is returned. Other possible error codes are:

PCRE2_ERROR_NOMEMORY

The buffer was too small for pcre2_substring_copy_bynumber(), or the
attempt to get memory failed for pcre2_substring_get_bynumber().

PCRE2_ERROR_NOSUBSTRING

There is no substring with that number in the pattern, that is, the number is
greater than the number of capturing parentheses.

PCRE2_ERROR_UNAVAILABLE

The substring number, though not greater than the number of captures in the
pattern, is greater than the number of slots in the ovector, so the substring
could not be captured.

PCRE2_ERROR_UNSET

The substring did not participate in the match. For example, if the pattern is
(abc)|(def) and the subject is "def", and the ovector contains at least two
capturing slots, substring number 1 is unset.
EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS

The pcre2_substring_list_get() function extracts all available substrings
and builds a list of pointers to them. It also (optionally) builds a second
list that contains their lengths (in code units), excluding a terminating zero
that is added to each of them. All this is done in a single block of memory
that is obtained using the same memory allocation function that was used to get
the match data block.

This function must be called only after a successful match. If called after a
partial match, the error code PCRE2_ERROR_PARTIAL is returned.

The address of the memory block is returned via listptr, which is also
the start of the list of string pointers. The end of the list is marked by a
NULL pointer. The address of the list of lengths is returned via
lengthsptr. If your strings do not contain binary zeros and you do not
therefore need the lengths, you may supply NULL as the lengthsptr
argument to disable the creation of a list of lengths. The yield of the
function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the memory block
could not be obtained. When the list is no longer needed, it should be freed by
calling pcre2_substring_list_free().

If this function encounters a substring that is unset, which can happen when
capturing subpattern number n+1 matches some part of the subject, but
subpattern n has not been used at all, it returns an empty string. This
can be distinguished from a genuine zero-length substring by inspecting the
appropriate offset in the ovector, which contain PCRE2_UNSET for unset
substrings, or by calling pcre2_substring_length_bynumber().

To extract a substring by name, you first have to find associated number.
For example, for this pattern:

(a+)b(?<xxx>\d+)...

the number of the subpattern called "xxx" is 2. If the name is known to be
unique (PCRE2_DUPNAMES was not set), you can find the number from the name by
calling pcre2_substring_number_from_name(). The first argument is the
compiled pattern, and the second is the name. The yield of the function is the
subpattern number, PCRE2_ERROR_NOSUBSTRING if there is no subpattern of that
name, or PCRE2_ERROR_NOUNIQUESUBSTRING if there is more than one subpattern of
that name. Given the number, you can extract the substring directly from the
ovector, or use one of the "bynumber" functions described above.

For convenience, there are also "byname" functions that correspond to the
"bynumber" functions, the only difference being that the second argument is a
name instead of a number. If PCRE2_DUPNAMES is set and there are duplicate
names, these functions scan all the groups with the given name, and return the
first named string that is set.

If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING is
returned. If all groups with the name have numbers that are greater than the
number of slots in the ovector, PCRE2_ERROR_UNAVAILABLE is returned. If there
is at least one group with a slot in the ovector, but no group is found to be
set, PCRE2_ERROR_UNSET is returned.

Warning: If the pattern uses the (?| feature to set up multiple
subpatterns with the same number, as described in the
section on duplicate subpattern numbers
in the
pcre2pattern
page, you cannot use names to distinguish the different subpatterns, because
names are not included in the compiled code. The matching process uses only
numbers. For this reason, the use of different names for subpatterns of the
same number causes an error at compile time.

This function calls pcre2_match() and then makes a copy of the subject
string in outputbuffer, replacing the part that was matched with the
replacement string, whose length is supplied in rlength. This can
be given as PCRE2_ZERO_TERMINATED for a zero-terminated string. Matches in
which a \K item in a lookahead in the pattern causes the match to end before
it starts are not supported, and give rise to an error return.

The first seven arguments of pcre2_substitute() are the same as for
pcre2_match(), except that the partial matching options are not
permitted, and match_data may be passed as NULL, in which case a match
data block is obtained and freed within this function, using memory management
functions from the match context, if provided, or else those that were used to
allocate memory for the compiled code.

The outlengthptr argument must point to a variable that contains the
length, in code units, of the output buffer. If the function is successful, the
value is updated to contain the length of the new string, excluding the
trailing zero that is automatically added.

If the function is not successful, the value set via outlengthptr depends
on the type of error. For syntax errors in the replacement string, the value is
the offset in the replacement string where the error was detected. For other
errors, the value is PCRE2_UNSET by default. This includes the case of the
output buffer being too small, unless PCRE2_SUBSTITUTE_OVERFLOW_LENGTH is set
(see below), in which case the value is the minimum length needed, including
space for the trailing zero. Note that in order to compute the required length,
pcre2_substitute() has to simulate all the matching and copying, instead
of giving an error return as soon as the buffer overflows. Note also that the
length is in code units, not bytes.

In the replacement string, which is interpreted as a UTF string in UTF mode,
and is checked for UTF validity unless the PCRE2_NO_UTF_CHECK option is set, a
dollar character is an escape character that can specify the insertion of
characters from capturing groups or (*MARK), (*PRUNE), or (*THEN) items in the
pattern. The following forms are always recognized:

$$ insert a dollar character
$<n> or ${<n>} insert the contents of group <n>
$*MARK or ${*MARK} insert a (*MARK), (*PRUNE), or (*THEN) name

Either a group number or a group name can be given for <n>. Curly brackets are
required only if the following character would be interpreted as part of the
number or name. The number may be zero to include the entire matched string.
For example, if the pattern a(b)c is matched with "=abc=" and the replacement
string "+$1$0$1+", the result is "=+babcb+=".

$*MARK inserts the name from the last encountered (*MARK), (*PRUNE), or (*THEN)
on the matching path that has a name. (*MARK) must always include a name, but
(*PRUNE) and (*THEN) need not. For example, in the case of (*MARK:A)(*PRUNE)
the name inserted is "A", but for (*MARK:A)(*PRUNE:B) the relevant name is "B".
This facility can be used to perform simple simultaneous substitutions, as this
pcre2test example shows:

As well as the usual options for pcre2_match(), a number of additional
options can be set in the options argument of pcre2_substitute().

PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject string,
replacing every matching substring. If this option is not set, only the first
matching substring is replaced. The search for matches takes place in the
original subject string (that is, previous replacements do not affect it).
Iteration is implemented by advancing the startoffset value for each
search, which is always passed the entire subject string. If an offset limit is
set in the match context, searching stops when that limit is reached.

You can restrict the effect of a global substitution to a portion of the
subject string by setting either or both of startoffset and an offset
limit. Here is a \fPpcre2test\fP example:

When continuing with global substitutions after matching a substring with zero
length, an attempt to find a non-empty match at the same offset is performed.
If this is not successful, the offset is advanced by one character except when
CRLF is a valid newline sequence and the next two characters are CR, LF. In
this case, the offset is advanced by two characters.

PCRE2_SUBSTITUTE_OVERFLOW_LENGTH changes what happens when the output buffer is
too small. The default action is to return PCRE2_ERROR_NOMEMORY immediately. If
this option is set, however, pcre2_substitute() continues to go through
the motions of matching and substituting (without, of course, writing anything)
in order to compute the size of buffer that is needed. This value is passed
back via the outlengthptr variable, with the result of the function still
being PCRE2_ERROR_NOMEMORY.

Passing a buffer size of zero is a permitted way of finding out how much memory
is needed for given substitution. However, this does mean that the entire
operation is carried out twice. Depending on the application, it may be more
efficient to allocate a large buffer and free the excess afterwards, instead of
using PCRE2_SUBSTITUTE_OVERFLOW_LENGTH.

PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capturing groups that do
not appear in the pattern to be treated as unset groups. This option should be
used with care, because it means that a typo in a group name or number no
longer causes the PCRE2_ERROR_NOSUBSTRING error.

PCRE2_SUBSTITUTE_UNSET_EMPTY causes unset capturing groups (including unknown
groups when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) to be treated as empty
strings when inserted as described above. If this option is not set, an attempt
to insert an unset group causes the PCRE2_ERROR_UNSET error. This option does
not influence the extended substitution syntax described below.

PCRE2_SUBSTITUTE_EXTENDED causes extra processing to be applied to the
replacement string. Without this option, only the dollar character is special,
and only the group insertion forms listed above are valid. When
PCRE2_SUBSTITUTE_EXTENDED is set, two things change:

Firstly, backslash in a replacement string is interpreted as an escape
character. The usual forms such as \n or \x{ddd} can be used to specify
particular character codes, and backslash followed by any non-alphanumeric
character quotes that character. Extended quoting can be coded using \Q...\E,
exactly as in pattern strings.

There are also four escape sequences for forcing the case of inserted letters.
The insertion mechanism has three states: no case forcing, force upper case,
and force lower case. The escape sequences change the current state: \U and
\L change to upper or lower case forcing, respectively, and \E (when not
terminating a \Q quoted sequence) reverts to no case forcing. The sequences
\u and \l force the next character (if it is a letter) to upper or lower
case, respectively, and then the state automatically reverts to no case
forcing. Case forcing applies to all inserted characters, including those from
captured groups and letters within \Q...\E quoted sequences.

Note that case forcing sequences such as \U...\E do not nest. For example,
the result of processing "\Uaa\LBB\Ecc\E" is "AAbbcc"; the final \E has no
effect.

The second effect of setting PCRE2_SUBSTITUTE_EXTENDED is to add more
flexibility to group substitution. The syntax is similar to that used by Bash:

${<n>:-<string>}
${<n>:+<string1>:<string2>}

As before, <n> may be a group number or a name. The first form specifies a
default value. If group <n> is set, its value is inserted; if not, <string> is
expanded and the result inserted. The second form specifies strings that are
expanded and inserted when group <n> is set or unset, respectively. The first
form is just a convenient shorthand for

${<n>:+${<n>}:<string>}

Backslash can be used to escape colons and closing curly brackets in the
replacement strings. A change of the case forcing state within a replacement
string remains in force afterwards, as shown in this pcre2test example:

The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these extended
substitutions. However, PCRE2_SUBSTITUTE_UNKNOWN_UNSET does cause unknown
groups in the extended syntax forms to be treated as unset.

If successful, pcre2_substitute() returns the number of replacements that
were made. This may be zero if no matches were found, and is never greater than
1 unless PCRE2_SUBSTITUTE_GLOBAL is set.

In the event of an error, a negative error code is returned. Except for
PCRE2_ERROR_NOMATCH (which is never returned), errors from pcre2_match()
are passed straight back.

PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring insertion,
unless PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set.

PCRE2_ERROR_UNSET is returned for an unset substring insertion (including an
unknown substring when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) when the simple
(non-extended) syntax is used and PCRE2_SUBSTITUTE_UNSET_EMPTY is not set.

PCRE2_ERROR_NOMEMORY is returned if the output buffer is not big enough. If the
PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is set, the size of buffer that is
needed is returned via outlengthptr. Note that this does not happen by
default.

PCRE2_ERROR_BADREPLACEMENT is used for miscellaneous syntax errors in the
replacement string, with more particular errors being PCRE2_ERROR_BADREPESCAPE
(invalid escape sequence), PCRE2_ERROR_REPMISSINGBRACE (closing curly bracket
not found), PCRE2_ERROR_BADSUBSTITUTION (syntax error in extended group
substitution), and PCRE2_ERROR_BADSUBSPATTERN (the pattern match ended before
it started, which can happen if \K is used in an assertion).

As for all PCRE2 errors, a text message that describes the error can be
obtained by calling the pcre2_get_error_message() function (see
"Obtaining a textual error message"
above).

When a pattern is compiled with the PCRE2_DUPNAMES option, names for
subpatterns are not required to be unique. Duplicate names are always allowed
for subpatterns with the same number, created by using the (?| feature. Indeed,
if such subpatterns are named, they are required to use the same names.

Normally, patterns with duplicate names are such that in any one match, only
one of the named subpatterns participates. An example is shown in the
pcre2pattern
documentation.

When duplicates are present, pcre2_substring_copy_byname() and
pcre2_substring_get_byname() return the first substring corresponding to
the given name that is set. Only if none are set is PCRE2_ERROR_UNSET is
returned. The pcre2_substring_number_from_name() function returns the
error PCRE2_ERROR_NOUNIQUESUBSTRING when there are duplicate names.

If you want to get full details of all captured substrings for a given name,
you must use the pcre2_substring_nametable_scan() function. The first
argument is the compiled pattern, and the second is the name. If the third and
fourth arguments are NULL, the function returns a group number for a unique
name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise.

When the third and fourth arguments are not NULL, they must be pointers to
variables that are updated by the function. After it has run, they point to the
first and last entries in the name-to-number table for the given name, and the
function returns the length of each entry in code units. In both cases,
PCRE2_ERROR_NOSUBSTRING is returned if there are no entries for the given name.

The format of the name table is described
above
in the section entitled Information about a pattern. Given all the
relevant entries for the name, you can extract each of their numbers, and hence
the captured data.

The traditional matching function uses a similar algorithm to Perl, which stops
when it finds the first match at a given point in the subject. If you want to
find all possible matches, or the longest possible match at a given position,
consider using the alternative matching function (see below) instead. If you
cannot use the alternative function, you can kludge it up by making use of the
callout facility, which is described in the
pcre2callout
documentation.

What you have to do is to insert a callout right at the end of the pattern.
When your callout function is called, extract and save the current matched
substring. Then return 1, which forces pcre2_match() to backtrack and try
other alternatives. Ultimately, when it runs out of matches,
pcre2_match() will yield PCRE2_ERROR_NOMATCH.

The function pcre2_dfa_match() is called to match a subject string
against a compiled pattern, using a matching algorithm that scans the subject
string just once (not counting lookaround assertions), and does not backtrack.
This has different characteristics to the normal algorithm, and is not
compatible with Perl. Some of the features of PCRE2 patterns are not supported.
Nevertheless, there are times when this kind of matching can be useful. For a
discussion of the two matching algorithms, and a list of features that
pcre2_dfa_match() does not support, see the
pcre2matching
documentation.

The arguments for the pcre2_dfa_match() function are the same as for
pcre2_match(), plus two extras. The ovector within the match data block
is used in a different way, and this is described below. The other common
arguments are used in the same way as for pcre2_match(), so their
description is not repeated here.

The two additional arguments provide workspace for the function. The workspace
vector should contain at least 20 elements. It is used for keeping track of
multiple paths through the pattern tree. More workspace is needed for patterns
and subjects where there are a lot of potential matches.

The unused bits of the options argument for pcre2_dfa_match() must
be zero. The only bits that may be set are PCRE2_ANCHORED, PCRE2_ENDANCHORED,
PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, PCRE2_PARTIAL_SOFT, PCRE2_DFA_SHORTEST,
and PCRE2_DFA_RESTART. All but the last four of these are exactly the same as
for pcre2_match(), so their description is not repeated here.

PCRE2_PARTIAL_HARD
PCRE2_PARTIAL_SOFT

These have the same general effect as they do for pcre2_match(), but the
details are slightly different. When PCRE2_PARTIAL_HARD is set for
pcre2_dfa_match(), it returns PCRE2_ERROR_PARTIAL if the end of the
subject is reached and there is still at least one matching possibility that
requires additional characters. This happens even if some complete matches have
already been found. When PCRE2_PARTIAL_SOFT is set, the return code
PCRE2_ERROR_NOMATCH is converted into PCRE2_ERROR_PARTIAL if the end of the
subject is reached, there have been no complete matches, but there is still at
least one matching possibility. The portion of the string that was inspected
when the longest partial match was found is set as the first matching string in
both cases. There is a more detailed discussion of partial and multi-segment
matching, with examples, in the
pcre2partial
documentation.

PCRE2_DFA_SHORTEST

Setting the PCRE2_DFA_SHORTEST option causes the matching algorithm to stop as
soon as it has found one match. Because of the way the alternative algorithm
works, this is necessarily the shortest possible match at the first possible
matching point in the subject string.

PCRE2_DFA_RESTART

When pcre2_dfa_match() returns a partial match, it is possible to call it
again, with additional subject characters, and have it continue with the same
match. The PCRE2_DFA_RESTART option requests this action; when it is set, the
workspace and wscount options must reference the same vector as
before because data about the match so far is left in them after a partial
match. There is more discussion of this facility in the
pcre2partial
documentation.
Successful returns from pcre2_dfa_match()

When pcre2_dfa_match() succeeds, it may have matched more than one
substring in the subject. Note, however, that all the matches from one run of
the function start at the same point in the subject. The shorter matches are
all initial substrings of the longer matches. For example, if the pattern

On success, the yield of the function is a number greater than zero, which is
the number of matched substrings. The offsets of the substrings are returned in
the ovector, and can be extracted by number in the same way as for
pcre2_match(), but the numbers bear no relation to any capturing groups
that may exist in the pattern, because DFA matching does not support group
capture.

Calls to the convenience functions that extract substrings by name
return the error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used after a
DFA match. The convenience functions that extract substrings by number never
return PCRE2_ERROR_NOSUBSTRING, and the meanings of some other errors are
slightly different:

PCRE2_ERROR_UNAVAILABLE

The ovector is not big enough to include a slot for the given substring number.

PCRE2_ERROR_UNSET

There is a slot in the ovector for this substring, but there were insufficient
matches to fill it.

The matched strings are stored in the ovector in reverse order of length; that
is, the longest matching string is first. If there were too many matches to fit
into the ovector, the yield of the function is zero, and the vector is filled
with the longest matches.

NOTE: PCRE2's "auto-possessification" optimization usually applies to character
repeats at the end of a pattern (as well as internally). For example, the
pattern "a\d+" is compiled as if it were "a\d++". For DFA matching, this
means that only one possible match is found. If you really do want multiple
matches in such cases, either use an ungreedy repeat such as "a\d+?" or set
the PCRE2_NO_AUTO_POSSESS option when compiling.

Error returns from pcre2_dfa_match()

The pcre2_dfa_match() function returns a negative number when it fails.
Many of the errors are the same as for pcre2_match(), as described
above.
There are in addition the following errors that are specific to
pcre2_dfa_match():

PCRE2_ERROR_DFA_UITEM

This return is given if pcre2_dfa_match() encounters an item in the
pattern that it does not support, for instance, the use of \C in a UTF mode or
a back reference.

PCRE2_ERROR_DFA_UCOND

This return is given if pcre2_dfa_match() encounters a condition item
that uses a back reference for the condition, or a test for recursion in a
specific group. These are not supported.

PCRE2_ERROR_DFA_WSSIZE

This return is given if pcre2_dfa_match() runs out of space in the
workspace vector.

PCRE2_ERROR_DFA_RECURSE

When a recursive subpattern is processed, the matching function calls itself
recursively, using private memory for the ovector and workspace. This
error is given if the internal ovector is not large enough. This should be
extremely rare, as a vector of size 1000 is used.

PCRE2_ERROR_DFA_BADRESTART

When pcre2_dfa_match() is called with the PCRE2_DFA_RESTART option,
some plausibility checks are made on the contents of the workspace, which
should contain data about the previous partial match. If any of these checks
fail, this error is given.
SEE ALSO